Pipeline Model Parallel (#1202)

* Init apex.ppu (pipeline model parallel utility)

Reference commit:

```
commit 5ab646376d67831601d5552c193241d017f1b35c (HEAD -> main, internal/main)
Merge: 14f2c684 7b293d9b
Author: Mohammad Shoeybi <mshoeybi@nvidia.com>
Date:   Wed Sep 22 22:57:54 2021 -0700

    Merge branch 'add_BOS' into 'main'

    Add Beginning of Sentence token option and adding semaphore while multi-threading to prevent crashes and hangs due to connection keep-alives

    See merge request ADLR/megatron-lm!328
```

* removing get_args and replace import - phase 1

* removing get_args and replace import - phase 2

* move ppu to apex.transformer.pipeline_parallel

* update two __init__.py

* update READMEs

* mpu -> parallel_state & tensor_parallel

* fix

* remove not pipeline files

* separate schedules.py - phase 1

* dissect schedules.py

* data_iterators -> batch

* remove optimizer from forward_backward_step funcs

* init test

* Apply 2 suggestion(s) to 2 file(s)

* fix cyclic import

* fix syntax of Callable

* fix - 1

* move directory as testing used for pp test as well

* add some functions for num microbatches calculator

* model is a list in pipeline parallel

* skip build num microbatch calculator

* fix test

* assert -> raise

* skip args printing

* specify tensor shape everywhere even if None - phase 1

* private timers

* passing tensor shape & dtype around

* update dtype handling by introducing helper func

* write helper func to reduce cyclomatic complexity

* remove duplicate

* update

* move split_tensor_into_1d_equal_chunks to avoid cyclic import

* tmp

* cosmetic

* move gather_split_1d_tensor to avoid cyclic imports

* remove debug print

* add outer loop

* early return if possible

* cosmetic

* passing around tensor shape

* refactor test

* add script to learn batch sampler behavior

* update

* minibatch splitter

* add minibatch splitter

* split minibatch into microbatches

* minor changes

* uncomment split batch for test sake

* set as attribute

* study the behavior of no pipelining

* debug 1

* reflect test util namespace change

* update readme

* cosmetic in test

* add model build helper func for interleaving shced

* adding model builder from megatron

* canbe cyclic import

* fix

* enable interleaving test, but failing even if forward only

* fix batch preparation

* add explanation

* print data parallel size

* fix typo

* Add Megatron style GPT model by Rishi
Co-authored-by: Rishi Puri <riship@nvidia.com>

* update

* type hint for jit

* fix forward_backward_no_pipelining test

* pipeline forward backward seem to hang if not forward only

* fix typo

* debug

* add p2p test

* simplify

* fix

* tentative

* set both tmp and pmp to 1

* init

* fix typo

* fix

* fix path of divide

* set seed for tmp

* update upon Eddie comment

* fix typo

* adding failing data loader test

* fix

* megatron still failing

* check in

* with the nested loop of new order, interleaving seems fine

* cosmetic change

* make `forward_backward_pipelining_with_interleaving private

* warn users that interleaving sched is unstable

* move noop handler to no pipelining

* comment out rank_print

* make `build_model` more flexible

* skip megatron test tentatively

* correctly comment out rank_print

* correctly comment out rank_print

* correctly comment out rank_print

* skip appropriately

* remove wip p2p comm test

* update type hint of model_provider_func

* disable tf32 in each test script

* skip interleaving w/ backward

* rename as mpu is the old name

* remove broken case

* expose build_model func

* delete `dist.ring_exchange` func call and `use_ring_exchange` argument

* nit fixes

* check in

* remove unused file

* update the list

* update tensor shape

* remove mixed dtype case

* use torch.distributed.run

* 2020 -> 2021

* another 2020 -> 2021

* docstring & type hint

* fix teardown

* update

* change to experimental

* check if warned
Co-authored-by: Rishi Puri <riship@nvidia.com>
Co-authored-by: Eddie Yan <eddiey@nvidia.com>

apex/__init__.py

 # May help avoid undefined symbol errors https://pytorch.org/cppdocs/notes/faq.html#undefined-symbol-errors-from-pytorch-aten
 import torch
-import warnings
 
 if torch.distributed.is_available():
     from . import parallel

apex/_autocast_utils.py

+from typing import Optional
+
 import torch
 
 
+def _get_current_dtype(dtype: Optional[torch.dtype] = None) -> torch.dtype:
+    if not torch.is_autocast_enabled():
+        return torch.float or dtype
+    else:
+        return torch.get_autocast_gpu_dtype()
+
+
 def _cast_if_autocast_enabled(*args):
     if not torch.is_autocast_enabled():
         return args

apex/transformer/README.md

@@ -2,4 +2,80 @@
 
 `apex.transformer` is a module which enables efficient large Transformer models at scale.
 
-`apex.transformer.tensor_parallel` is based on [NVIDIA/Megatron-LM](https://github.com/NVIDIA/Megatron-LM)'s `megatron.mpu` module.
+`apex.transformer.tensor_parallel` and `apex.transformer.pipeline_parallel` are both based on [NVIDIA/Megatron-LM](https://github.com/NVIDIA/Megatron-LM)'s module.
+The former is based on `megatron.mpu` and the latter is on `megatron.schedules` and `megatron.p2p_communication`.
+
+## Tensor Model Parallel (TP)
+
+APEX's tensor model parallel utilities provides some `torch.nn.Module`'s, custom fused kernels, and PRNG state handling.
+See Appendix B.2 of [Megatron-LM: Training Multi-Billion Parameter Language Models Using Model Parallelism](https://arxiv.org/abs/1909.08053) for the details of
+PRNG state handling.
+
+## Pipeline Model Parallel (PP)
+APEX's pipeline model parallel functions require models to have `.set_input_tensor` because
+the input tensor for `.forward` method can be `None`.
+
+The following is a really casual sketch of training script with apex pp.
+
+```python
+import torch
+import torch.nn as nn
+import torch.nn.functional as F
+
+from apex.transformer import parallel_state
+from apex.transformer.pipeline_parallel import get_forward_backward_func
+
+
+class Model(nn.Module):
+
+    ...
+
+    def __init__(self, *args, **kwargs):
+        super().__init__()
+        pre_process = kwargs.pop("pre_process")
+        post_process = kwargs.pop("post_process")
+
+    def set_input_tensor(self, tensor):
+        self.input_tensor = tensor
+
+    def forward(self, x, ...):
+        if parallel_state.is_pipeline_first_stage():
+            input = x
+        else:
+            input = self.input_tensor
+        ...
+
+
+def model_provider_func(*args, **kwargs):
+    return Model(*args, **kwargs)
+
+
+def loss_func(pred, label):
+    loss = ...
+    averaged_loss = average_losses_across_data_parallel_group([loss])
+    return loss, {'nice_loss': averaged_loss}
+
+
+def forward_step_func(batch, model):
+    input, label = process_batch(batch)
+    out = model(input)
+    return out, partial(loss_func, label)
+
+
+forward_backward_func = get_forward_backward_func(virtual_pipeline_model_parallel_size, pipeline_model_parallel_size)
+
+
+parallel_state.initialize_model_parallel(
+    tensor_model_parallel_size,
+    pipeline_model_parallel_size,
+    virtual_pipeline_model_parallel_size,
+)
+# The following line basically is equivalent to `build_model(Model, wrap_with_ddp, virtual_pipeline_model_parallel_size, *model_args, **model_kwargs)`
+model = build_model(model_provider_func, wrap_with_ddp, virtual_pipeline_model_parallel_size, *model_args, **model_kwargs)
+optimizer = ...
+data_loader = ...
+for epoch in range(num_epochs):
+    for batch in data_loader:
+        forward_backward_func(forward_step_func, batch, model, forward_only=False, tensor_shape)
+        optimizer.step()
+```

apex/transformer/__init__.py

-from . import tensor_parallel
-from . import functional
-from .enums import LayerType
-from .enums import AttnType
-from .enums import AttnMaskType
-from .parallel_state import (
-    is_unitialized,
-    destroy_model_parallel,
-    get_data_parallel_group,
-    get_data_parallel_rank,
-    get_data_parallel_world_size,
-    get_embedding_group,
-    get_model_parallel_group,
-    get_tensor_model_parallel_group,
-    get_pipeline_model_parallel_group,
-    get_tensor_model_parallel_rank,
-    set_tensor_model_parallel_rank,
-    get_pipeline_model_parallel_rank,
-    set_pipeline_model_parallel_rank,
-    is_pipeline_first_stage,
-    is_pipeline_last_stage,
-    get_tensor_model_parallel_src_rank,
-    get_pipeline_model_parallel_first_rank,
-    get_pipeline_model_parallel_last_rank,
-    get_pipeline_model_parallel_next_rank,
-    get_pipeline_model_parallel_prev_rank,
-    get_tensor_model_parallel_world_size,
-    set_tensor_model_parallel_world_size,
-    get_pipeline_model_parallel_world_size,
-    set_pipeline_model_parallel_world_size,
-    get_virtual_pipeline_model_parallel_rank,
-    set_virtual_pipeline_model_parallel_rank,
-    initialize_model_parallel,
-    model_parallel_is_initialized,
-)
+from apex.transformer import functional
+from apex.transformer import parallel_state
+from apex.transformer import pipeline_parallel
+from apex.transformer import tensor_parallel
+from apex.transformer import utils
+from apex.transformer.enums import LayerType
+from apex.transformer.enums import AttnType
+from apex.transformer.enums import AttnMaskType
+
+
+__all__ = [
+    "functional",
+    "parallel_state",
+    "pipeline_parallel",
+    "tensor_parallel",
+    "utils",
+    # enums.py
+    "LayerType",
+    "AttnType",
+    "AttnMaskType",
+]

apex/transformer/enums.py

 # coding=utf-8
-# Copyright (c) 2020, NVIDIA CORPORATION.  All rights reserved.
+# Copyright (c) 2021, NVIDIA CORPORATION.  All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.

apex/transformer/functional/__init__.py

-from .fused_softmax import FusedScaleMaskSoftmax
+from apex.transformer.functional.fused_softmax import FusedScaleMaskSoftmax
 
 __all__ = [
     "FusedScaleMaskSoftmax",

apex/transformer/functional/fused_softmax.py

 # coding=utf-8
-# Copyright (c) 2020, NVIDIA CORPORATION.  All rights reserved.
+# Copyright (c) 2021, NVIDIA CORPORATION.  All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -15,7 +15,7 @@
 import torch
 
 from apex._autocast_utils import _cast_if_autocast_enabled
-from ..enums import AttnMaskType
+from apex.transformer.enums import AttnMaskType
 
 
 class ScaledUpperTriangMaskedSoftmax(torch.autograd.Function):

apex/transformer/tensor_parallel/microbatches.py → apex/transformer/microbatches.py

 # coding=utf-8
-# Copyright (c) 2020, NVIDIA CORPORATION.  All rights reserved.
+# Copyright (c) 2021, NVIDIA CORPORATION.  All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -15,35 +15,41 @@
 """Megatron number of micro-batches calculators."""
 from abc import ABC
 from abc import abstractmethod
+from typing import Optional, List
 
 
-def build_num_microbatches_calculator(args):
-
+def build_num_microbatches_calculator(
+        rank: int,
+        rampup_batch_size: Optional[List[int]],
+        global_batch_size: int,
+        micro_batch_size: int,
+        data_parallel_size: int,
+):
     # Constant num micro-batches.
-    if args.rampup_batch_size is None:
+    if rampup_batch_size is None:
         num_microbatches_calculator = ConstantNumMicroBatches(
-            args.global_batch_size, args.micro_batch_size, args.data_parallel_size
+            global_batch_size, micro_batch_size, data_parallel_size
         )
-        if args.rank == 0:
+        if rank == 0:
             print(
                 "setting number of micro-batches to constant {}".format(num_microbatches_calculator.get()), flush=True
             )
 
     else:
-        assert len(args.rampup_batch_size) == 3, (
+        assert len(rampup_batch_size) == 3, (
             "expected the following "
             "format: --rampup-batch-size <start batch size> "
             "<batch size incerement> <ramp-up samples>"
         )
-        start_batch_size = int(args.rampup_batch_size[0])
-        batch_size_increment = int(args.rampup_batch_size[1])
-        ramup_samples = int(args.rampup_batch_size[2])
-        if args.rank == 0:
+        start_batch_size = int(rampup_batch_size[0])
+        batch_size_increment = int(rampup_batch_size[1])
+        ramup_samples = int(rampup_batch_size[2])
+        if rank == 0:
             print(
                 "will use batch size rampup starting from global batch "
                 "size {} to global batch size {} with batch size increments "
                 "{} over {} samples.".format(
-                    start_batch_size, args.global_batch_size, batch_size_increment, ramup_samples
+                    start_batch_size, global_batch_size, batch_size_increment, ramup_samples
                 ),
                 flush=True,
             )
@@ -51,9 +57,9 @@ def build_num_microbatches_calculator(args):
             start_batch_size,
             batch_size_increment,
             ramup_samples,
-            args.global_batch_size,
-            args.micro_batch_size,
-            args.data_parallel_size,
+            global_batch_size,
+            micro_batch_size,
+            data_parallel_size,
         )
 
     return num_microbatches_calculator
@@ -86,6 +92,8 @@ class ConstantNumMicroBatches(NumMicroBatchesCalculator):
         assert self.num_micro_batches >= 1
         self.current_global_batch_size = global_batch_size
 
+        self.micro_batch_size = micro_batch_size
+
     def update(self, consumed_samples, consistency_check):
         pass
 

apex/transformer/parallel_state.py

 # coding=utf-8
-# Copyright (c) 2020, NVIDIA CORPORATION.  All rights reserved.
+# Copyright (c) 2021, NVIDIA CORPORATION.  All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -17,7 +17,7 @@ import torch
 
 # TODO (mkozuki): Consider dissecting utils as this utils import is here
 # only for ensure_divisibility
-from .tensor_parallel import utils
+from apex.transformer.utils import ensure_divisibility
 
 
 # Intra-layer model parallel group that the current rank belongs to.
@@ -76,17 +76,17 @@ def initialize_model_parallel(
     with a total of 16 GPUs, rank 0 to 7 belong to the first box and
     ranks 8 to 15 belong to the second box.
     """
-    if torch.distributed.get_rank() == 0:
-        print("> initializing tensor model parallel with size {}".format(tensor_model_parallel_size_))
-        print("> initializing pipeline model parallel with size {}".format(pipeline_model_parallel_size_))
     # Get world size and rank. Ensure some consistencies.
     assert torch.distributed.is_initialized()
     world_size = torch.distributed.get_world_size()
     tensor_model_parallel_size = min(tensor_model_parallel_size_, world_size)
     pipeline_model_parallel_size = min(pipeline_model_parallel_size_, world_size)
-    # TODO (mkozuki): Consider moving `ensure_divisibility` to this file.
-    utils.ensure_divisibility(world_size, tensor_model_parallel_size * pipeline_model_parallel_size)
+    ensure_divisibility(world_size, tensor_model_parallel_size * pipeline_model_parallel_size)
     data_parallel_size = world_size // (tensor_model_parallel_size * pipeline_model_parallel_size)
+    if torch.distributed.get_rank() == 0:
+        print("> initializing tensor model parallel with size {}".format(tensor_model_parallel_size))
+        print("> initializing pipeline model parallel with size {}".format(pipeline_model_parallel_size))
+        print("> initializing data parallel with size {}".format(data_parallel_size))
 
     num_tensor_model_parallel_groups = world_size // tensor_model_parallel_size
     num_pipeline_model_parallel_groups = world_size // pipeline_model_parallel_size
@@ -344,3 +344,15 @@ def destroy_model_parallel():
     _DATA_PARALLEL_GROUP = None
     global _EMBEDDING_GROUP
     _EMBEDDING_GROUP = None
+    global _VIRTUAL_PIPELINE_MODEL_PARALLEL_RANK
+    _VIRTUAL_PIPELINE_MODEL_PARALLEL_RANK = None
+    global _VIRTUAL_PIPELINE_MODEL_PARALLEL_WORLD_SIZE
+    _VIRTUAL_PIPELINE_MODEL_PARALLEL_WORLD_SIZE = None
+    global _MPU_TENSOR_MODEL_PARALLEL_WORLD_SIZE
+    _MPU_TENSOR_MODEL_PARALLEL_WORLD_SIZE = None
+    global _MPU_PIPELINE_MODEL_PARALLEL_WORLD_SIZE
+    _MPU_PIPELINE_MODEL_PARALLEL_WORLD_SIZE = None
+    global _MPU_TENSOR_MODEL_PARALLEL_RANK
+    _MPU_TENSOR_MODEL_PARALLEL_RANK = None
+    global _MPU_PIPELINE_MODEL_PARALLEL_RANK
+    _MPU_PIPELINE_MODEL_PARALLEL_RANK = None

apex/transformer/pipeline_parallel/__init__.py

+from apex.transformer.pipeline_parallel.schedules import get_forward_backward_func
+from apex.transformer.pipeline_parallel.schedules.common import build_model
+
+
+__all__ = [
+    "get_forward_backward_func",
+    "build_model",
+]

apex/transformer/pipeline_parallel/_timers.py

+import time
+
+import torch
+
+
+class _Timer:
+    """Timer."""
+
+    def __init__(self, name):
+        self.name_ = name
+        self.elapsed_ = 0.0
+        self.started_ = False
+        self.start_time = time.time()
+
+    def start(self):
+        """Start the timer."""
+        assert not self.started_, "timer has already been started"
+        torch.cuda.synchronize()
+        self.start_time = time.time()
+        self.started_ = True
+
+    def stop(self):
+        """Stop the timer."""
+        assert self.started_, "timer is not started"
+        torch.cuda.synchronize()
+        self.elapsed_ += time.time() - self.start_time
+        self.started_ = False
+
+    def reset(self):
+        """Reset timer."""
+        self.elapsed_ = 0.0
+        self.started_ = False
+
+    def elapsed(self, reset=True):
+        """Calculate the elapsed time."""
+        started_ = self.started_
+        # If the timing in progress, end it first.
+        if self.started_:
+            self.stop()
+        # Get the elapsed time.
+        elapsed_ = self.elapsed_
+        # Reset the elapsed time
+        if reset:
+            self.reset()
+        # If timing was in progress, set it back.
+        if started_:
+            self.start()
+        return elapsed_
+
+
+class _Timers:
+    """Group of timers."""
+
+    def __init__(self):
+        self.timers = {}
+
+    def __call__(self, name):
+        if name not in self.timers:
+            self.timers[name] = _Timer(name)
+        return self.timers[name]
+
+    def write(self, names, writer, iteration, normalizer=1.0, reset=False):
+        """Write timers to a tensorboard writer"""
+        # currently when using add_scalars,
+        # torch.utils.add_scalars makes each timer its own run, which
+        # polutes the runs list, so we just add each as a scalar
+        assert normalizer > 0.0
+        for name in names:
+            value = self.timers[name].elapsed(reset=reset) / normalizer
+            writer.add_scalar(name + "-time", value, iteration)
+
+    def log(self, names, normalizer=1.0, reset=True):
+        """Log a group of timers."""
+        assert normalizer > 0.0
+        string = "time (ms)"
+        for name in names:
+            elapsed_time = self.timers[name].elapsed(reset=reset) * 1000.0 / normalizer
+            string += " | {}: {:.2f}".format(name, elapsed_time)
+        if torch.distributed.is_initialized():
+            if torch.distributed.get_rank() == (torch.distributed.get_world_size() - 1):
+                print(string, flush=True)
+        else:
+            print(string, flush=True)

apex/transformer/pipeline_parallel/p2p_communication.py

+# coding=utf-8
+# Copyright (c) 2021, NVIDIA CORPORATION.  All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from functools import reduce
+import operator
+from typing import Union, Optional, Tuple
+
+import torch
+
+from apex._autocast_utils import _get_current_dtype
+from apex.transformer import parallel_state
+from apex.transformer.utils import split_tensor_into_1d_equal_chunks
+from apex.transformer.utils import gather_split_1d_tensor
+from apex.transformer.pipeline_parallel.utils import Shape
+from apex.transformer.pipeline_parallel._timers import _Timers
+
+
+def _run_p2pops(
+        tensor_send_prev: Union[torch.Tensor, None],
+        tensor_send_next: Union[torch.Tensor, None],
+        tensor_recv_prev: Union[torch.Tensor, None],
+        tensor_recv_next: Union[torch.Tensor, None],
+):
+    ops = []
+    if tensor_send_prev is not None:
+        send_prev_op = torch.distributed.P2POp(
+            torch.distributed.isend,
+            tensor_send_prev,
+            parallel_state.get_pipeline_model_parallel_prev_rank(),
+        )
+        ops.append(send_prev_op)
+    if tensor_recv_prev is not None:
+        recv_prev_op = torch.distributed.P2POp(
+            torch.distributed.irecv,
+            tensor_recv_prev,
+            parallel_state.get_pipeline_model_parallel_prev_rank(),
+        )
+        ops.append(recv_prev_op)
+    if tensor_send_next is not None:
+        send_next_op = torch.distributed.P2POp(
+            torch.distributed.isend,
+            tensor_send_next,
+            parallel_state.get_pipeline_model_parallel_next_rank(),
+        )
+        ops.append(send_next_op)
+    if tensor_recv_next is not None:
+        recv_next_op = torch.distributed.P2POp(
+            torch.distributed.irecv,
+            tensor_recv_next,
+            parallel_state.get_pipeline_model_parallel_next_rank(),
+        )
+        ops.append(recv_next_op)
+    if len(ops) > 0:
+        reqs = torch.distributed.batch_isend_irecv(ops)
+        for req in reqs:
+            req.wait()
+
+
+# NOTE (mkozuki): Leaving `params_dytpe` as it is for future development in PyTorch, especially APEX O2 style AMP.
+# But as of v1.10, basically all tensors are torch.float32 except for output tensors of `autocast` compatible layers.
+def _communicate(
+    tensor_send_next: Optional[torch.Tensor],
+    tensor_send_prev: Optional[torch.Tensor],
+    recv_prev: bool,
+    recv_next: bool,
+    tensor_shape: Optional[Shape] = None,
+    override_scatter_gather_tensors_in_pipeline: bool = False,
+    dtype_: torch.dtype = torch.float,
+    *,
+    scatter_gather_tensors_in_pipeline: bool = True,
+    params_dtype: Optional[torch.dtype] = None,
+    fp32_residual_connection: bool = False,
+) -> Tuple[Union[torch.Tensor, None], Union[torch.Tensor, None]]:
+    """Base function for communication of tensors between stages.
+
+    Args:
+        tensor_send_next: tensor to send to next rank (no tensor sent if set to None).
+        tensor_send_prev: tensor to send to prev rank (no tensor sent if set to None).
+        recv_prev: boolean for whether tensor should be received from previous rank.
+        recv_next: boolean for whether tensor should be received from next rank.
+        tensor_shape: optional, use when the input sequence contains less tokens than the default sequence length
+        override_scatter_gather_tensors_in_pipeline:
+            optional, this is used when tensor_shape is provided to override scatter gather tensors
+        dtype_: This is used when tensor_shape is provided and what is the type of tensor_shape
+
+    Keyword args:
+        scatter_gather_tensors_in_pipeline: Optional. If :obj:`True`, use scatter/gather to optimize communication of tensors.
+        params_dtype: Optional and legacy. Defaults to torch.float. If you manually call `.half()` or `.bfloat16()` on
+            your model deliberately, pass this argument.
+        fp32_residual_connection: Optional. If :obj:`True`, move residual connections to fp32.
+
+    Returns:
+        tuple containing
+
+        - tensor_recv_prev: `torch.Tensor` if `recv_prev` is :obj:`True`, `None` otherwise.
+        - tensor_recv_next: `torch.Tensor` if `recv_next` is :obj:`True`, `None` otherwise.
+    """
+    # Create placeholder tensors for receive in forward and backward directions if needed.
+    tensor_recv_prev = None
+    tensor_recv_next = None
+    if tensor_shape is None:
+        # In megatron, `tensor_shape` is set to `(args.seq_length, args.micro_batch_size, args.hidden_size)`
+        raise RuntimeError(
+            "`tensor_shape` must be specified. Common `tensor_shape` is `(seq_length, micro_batch_size, hidden_size)`")
+    if not override_scatter_gather_tensors_in_pipeline and scatter_gather_tensors_in_pipeline:
+        tensor_chunk_shape = (reduce(operator.mul, tensor_shape, 1) // parallel_state.get_tensor_model_parallel_world_size(),)
+    else:
+        tensor_chunk_shape = tensor_shape
+    dtype = params_dtype or torch.float
+    if fp32_residual_connection:
+        dtype = torch.float
+
+    requires_grad = True
+    if dtype_ is not None:
+        dtype = dtype_
+        requires_grad = False
+
+    if recv_prev:
+        tensor_recv_prev = torch.empty(
+            tensor_chunk_shape,
+            requires_grad=requires_grad,
+            device=torch.cuda.current_device(),
+            dtype=dtype,
+        )
+    if recv_next:
+        tensor_recv_next = torch.empty(
+            tensor_chunk_shape,
+            requires_grad=requires_grad,
+            device=torch.cuda.current_device(),
+            dtype=dtype,
+        )
+
+    # Split tensor into smaller chunks if using scatter-gather optimization.
+    if not override_scatter_gather_tensors_in_pipeline and scatter_gather_tensors_in_pipeline:
+        if tensor_send_next is not None:
+            tensor_send_next = split_tensor_into_1d_equal_chunks(tensor_send_next)
+
+        if tensor_send_prev is not None:
+            tensor_send_prev = split_tensor_into_1d_equal_chunks(tensor_send_prev)
+
+    # Send tensors in both the forward and backward directions as appropriate.
+    _run_p2pops(tensor_send_prev, tensor_send_next, tensor_recv_prev, tensor_recv_next)
+    # To protect against race condition when using batch_isend_irecv().
+    torch.cuda.synchronize()
+
+    # If using scatter-gather optimization, gather smaller chunks.
+    if not override_scatter_gather_tensors_in_pipeline and scatter_gather_tensors_in_pipeline:
+        if recv_prev:
+            tensor_recv_prev = (
+                gather_split_1d_tensor(tensor_recv_prev)
+                .view(tensor_shape)
+                .requires_grad_()
+            )
+
+        if recv_next:
+            tensor_recv_next = (
+                gather_split_1d_tensor(tensor_recv_next)
+                .view(tensor_shape)
+                .requires_grad_()
+            )
+
+    return tensor_recv_prev, tensor_recv_next
+
+
+def recv_forward(
+        tensor_shape: Shape,
+        override_scatter_gather_tensors_in_pipeline: bool = False,
+        *,
+        dtype: Optional[torch.dtype] = None,
+        timers: _Timers = None,
+) -> torch.Tensor:
+    """Receive tensor from previous rank in pipeline (forward receive)."""
+    if parallel_state.is_pipeline_first_stage():
+        return None
+    if timers is not None:
+        timers("forward-recv").start()
+    input_tensor, _ = _communicate(
+        tensor_send_next=None,
+        tensor_send_prev=None,
+        recv_prev=True,
+        recv_next=False,
+        tensor_shape=tensor_shape,
+        override_scatter_gather_tensors_in_pipeline=override_scatter_gather_tensors_in_pipeline,
+        dtype_=_get_current_dtype(dtype),
+    )
+    if timers is not None:
+        timers("forward-recv").stop()
+    return input_tensor
+
+
+def recv_backward(
+        tensor_shape: Shape = None,
+        *,
+        dtype: Optional[torch.dtype] = None,
+        timers: _Timers = None,
+):
+    """Receive tensor from next rank in pipeline (backward receive)."""
+    if parallel_state.is_pipeline_last_stage():
+        return None
+    if timers is not None:
+        timers("backward-recv").start()
+    _, output_tensor_grad = _communicate(
+        tensor_send_next=None,
+        tensor_send_prev=None,
+        recv_prev=False,
+        recv_next=True,
+        tensor_shape=tensor_shape,
+        dtype_=_get_current_dtype(dtype),
+    )
+    if timers is not None:
+        timers("backward-recv").stop()
+    return output_tensor_grad
+
+
+def send_forward(
+        output_tensor: torch.Tensor,
+        override_scatter_gather_tensors_in_pipeline: bool = False,
+        tensor_shape: Shape = None,
+        *,
+        dtype: Optional[torch.dtype] = None,
+        timers: _Timers = None,
+) -> None:
+    """Send tensor to next rank in pipeline (forward send)."""
+    if parallel_state.is_pipeline_last_stage():
+        return
+    if timers is not None:
+        timers("forward-send").start()
+    _communicate(
+        tensor_send_next=output_tensor,
+        tensor_send_prev=None,
+        recv_prev=False,
+        recv_next=False,
+        override_scatter_gather_tensors_in_pipeline=override_scatter_gather_tensors_in_pipeline,
+        tensor_shape=tensor_shape,
+        dtype_=_get_current_dtype(dtype),
+    )
+    if timers is not None:
+        timers("forward-send").stop()
+
+
+def send_backward(
+        input_tensor_grad: torch.Tensor,
+        tensor_shape: Shape,
+        *,
+        dtype: Optional[torch.dtype] = None,
+        timers: _Timers = None,
+) -> None:
+    """Send tensor to previous rank in pipeline (backward send)."""
+    if parallel_state.is_pipeline_first_stage():
+        return
+    if timers is not None:
+        timers("backward-send").start()
+    _communicate(
+        tensor_send_next=None,
+        tensor_send_prev=input_tensor_grad,
+        recv_prev=False,
+        recv_next=False,
+        tensor_shape=tensor_shape,
+        dtype_=_get_current_dtype(dtype),
+    )
+    if timers is not None:
+        timers("backward-send").stop()
+
+
+def send_forward_recv_backward(
+        output_tensor: torch.Tensor,
+        tensor_shape: Shape,
+        *,
+        dtype: Optional[torch.dtype] = None,
+        timers: _Timers = None,
+) -> None:
+    """Batched send and recv with next rank in pipeline."""
+    if parallel_state.is_pipeline_last_stage():
+        return None
+    if timers is not None:
+        timers("forward-send-backward-recv").start()
+    _, output_tensor_grad = _communicate(
+        tensor_send_next=output_tensor,
+        tensor_send_prev=None,
+        recv_prev=False,
+        recv_next=True,
+        tensor_shape=tensor_shape,
+        dtype_=_get_current_dtype(dtype),
+    )
+    if timers is not None:
+        timers("forward-send-backward-recv").stop()
+    return output_tensor_grad
+
+
+def send_backward_recv_forward(
+        input_tensor_grad: torch.Tensor,
+        tensor_shape: Shape,
+        *,
+        dtype: Optional[torch.dtype] = None,
+        timers: _Timers = None,
+) -> torch.Tensor:
+    """Batched send and recv with previous rank in pipeline."""
+    if parallel_state.is_pipeline_first_stage():
+        return None
+    if timers is not None:
+        timers("backward-send-forward-recv").start()
+    input_tensor, _ = _communicate(
+        tensor_send_next=None,
+        tensor_send_prev=input_tensor_grad,
+        recv_prev=True,
+        recv_next=False,
+        tensor_shape=tensor_shape,
+        dtype_=_get_current_dtype(dtype),
+    )
+    if timers is not None:
+        timers("backward-send-forward-recv").stop()
+    return input_tensor
+
+
+def send_forward_recv_forward(
+        output_tensor: torch.Tensor,
+        recv_prev: bool,
+        tensor_shape: Shape,
+        *,
+        dtype: Optional[torch.dtype] = None,
+        timers: _Timers = None,
+) -> torch.Tensor:
+    """Batched recv from previous rank and send to next rank in pipeline."""
+    if timers is not None:
+        timers("forward-send-forward-recv").start()
+    input_tensor, _ = _communicate(
+        tensor_send_next=output_tensor,
+        tensor_send_prev=None,
+        recv_prev=recv_prev,
+        recv_next=False,
+        tensor_shape=tensor_shape,
+        dtype_=_get_current_dtype(dtype),
+    )
+    if timers is not None:
+        timers("forward-send-forward-recv").stop()
+    return input_tensor
+
+
+def send_backward_recv_backward(
+        input_tensor_grad: torch.Tensor,
+        recv_next: bool,
+        tensor_shape: Shape,
+        *,
+        dtype: torch.dtype = torch.float,
+        timers: _Timers = None,
+) -> torch.Tensor:
+    """Batched recv from next rank and send to previous rank in pipeline."""
+    if timers is not None:
+        timers("backward-send-backward-recv").start()
+    _, output_tensor_grad = _communicate(
+        tensor_send_next=None,
+        tensor_send_prev=input_tensor_grad,
+        recv_prev=False,
+        recv_next=recv_next,
+        tensor_shape=tensor_shape,
+        dtype_=_get_current_dtype(dtype),
+    )
+    if timers is not None:
+        timers("backward-send-backward-recv").stop()
+    return output_tensor_grad
+
+
+def send_forward_backward_recv_forward_backward(
+        output_tensor: torch.Tensor,
+        input_tensor_grad: torch.Tensor,
+        recv_prev: bool,
+        recv_next: bool,
+        tensor_shape: Shape,
+        *,
+        dtype: Optional[torch.dtype] = None,
+        timers: _Timers = None,
+):
+    """Batched send and recv with previous and next ranks in pipeline."""
+    if timers is not None:
+        timers("forward-backward-send-forward-backward-recv").start()
+    input_tensor, output_tensor_grad = _communicate(
+        tensor_send_next=output_tensor,
+        tensor_send_prev=input_tensor_grad,
+        recv_prev=recv_prev,
+        recv_next=recv_next,
+        tensor_shape=tensor_shape,
+        dtype_=_get_current_dtype(dtype),
+    )
+    if timers is not None:
+        timers("forward-backward-send-forward-backward-recv").stop()
+    return input_tensor, output_tensor_grad

apex/transformer/pipeline_parallel/schedules/__init__.py

+import warnings
+
+from apex.transformer import parallel_state
+from apex.transformer.pipeline_parallel.utils import get_num_microbatches
+from apex.transformer.pipeline_parallel.schedules.fwd_bwd_no_pipelining import forward_backward_no_pipelining
+from apex.transformer.pipeline_parallel.schedules.fwd_bwd_pipelining_with_interleaving import _forward_backward_pipelining_with_interleaving
+from apex.transformer.pipeline_parallel.schedules.fwd_bwd_pipelining_without_interleaving import (
+    forward_backward_pipelining_without_interleaving,
+)
+
+
+class ExperimentalWarning(Warning):
+    pass
+
+
+def get_forward_backward_func(
+    virtual_pipeline_model_parallel_size, pipeline_model_parallel_size,
+):
+    if parallel_state.get_pipeline_model_parallel_world_size() > 1:
+        if virtual_pipeline_model_parallel_size is not None:
+            if get_num_microbatches() % pipeline_model_parallel_size != 0:
+                msg = "number of microbatches is not divisible by pipeline-parallel size when using interleaved schedule"
+                raise RuntimeError(msg)
+            warnings.warn(
+                "Pipeline Model Parallel with interleaving scheduling is experimental. "
+                f"To use Pipeline Parallel without interleaving, set `virtual_pipeline_model_parallel_size` to `None`: {virtual_pipeline_model_parallel_size}",
+                ExperimentalWarning
+            )
+            forward_backward_func = _forward_backward_pipelining_with_interleaving
+        else:
+            forward_backward_func = forward_backward_pipelining_without_interleaving
+    else:
+        forward_backward_func = forward_backward_no_pipelining
+    return forward_backward_func
+
+
+__all__ = [
+    "get_forward_backward_func",
+]

apex/transformer/pipeline_parallel/schedules/common.py

+# NOTE (mkozuki): For simplicity, tentatively `timers` related operations are commented out.
+from typing import Any, Callable, Dict, List, Tuple, Union, Optional
+
+import torch
+
+from apex.transformer import parallel_state
+from apex.transformer.pipeline_parallel.utils import get_num_microbatches
+from apex.transformer.pipeline_parallel.utils import listify_model
+from apex.transformer.pipeline_parallel.utils import unwrap_model
+from apex.transformer.tensor_parallel.layers import set_defaults_if_not_set_tensor_model_parallel_attributes
+
+
+Batch = Union[torch.Tensor, List[torch.Tensor], Tuple[torch.Tensor, ...]]
+LossFunc = Callable[[torch.Tensor], torch.Tensor]
+FwdStepFunc = Callable[[Batch, torch.nn.Module], Tuple[torch.Tensor, LossFunc]]
+
+
+def build_model(
+        model_provider_func: Callable[[Any, Dict[str, Any]], torch.nn.Module],
+        wrap_with_ddp: bool = True,
+        virtual_pipeline_model_parallel_size: Optional[int] = None,
+        *args,
+        **kwargs
+) -> List[torch.nn.Module]:
+    """Build the model satisfying pipeline model parallel requirements.
+
+    This function sets `pre_process` and `post_process` to `**kwargs` and pass `*args` and `**kwargs` to
+    `model_provider_func`.
+
+    Args:
+        model_provider_func: A function which takes `*args` and `**kwargs` and returns a `nn.Module`.
+        wrap_with_ddp: If :obj:`True`, wrap the instantiated model
+            with `torch.nn.parallel.distributed.DistributedDataParallel`, a.k.a. `DDP`.
+        virtual_pipeline_model_parallel_size: Specify when using interleaving scheduling pipeline model parallel.
+        *args: arguments for model provider func
+        **kwargs: Keyword arguments for model provider func
+
+    Returns:
+        a list of `nn.Module`(s). If `virtual_pipeline_model_parallel_size` is not None,
+        the list has multiple models, otherwise one.
+    """
+    if (
+            parallel_state.get_pipeline_model_parallel_world_size() > 1 and
+            virtual_pipeline_model_parallel_size is not None
+    ):
+        model = []
+        for i in range(virtual_pipeline_model_parallel_size):
+            cur_args = args
+            cur_kwargs = kwargs
+            parallel_state.set_virtual_pipeline_model_parallel_rank(i)
+            # Set pre_process and post_process only after virtual rank is set.
+            pre_process = parallel_state.is_pipeline_first_stage()
+            post_process = parallel_state.is_pipeline_last_stage()
+            cur_kwargs.update({
+                "pre_process": pre_process,
+                "post_process": post_process,
+            })
+            this_model = model_provider_func(*cur_args, **cur_kwargs)
+            model.append(this_model)
+    else:
+        cur_args = args
+        cur_kwargs = kwargs
+        pre_process = parallel_state.is_pipeline_first_stage()
+        post_process = parallel_state.is_pipeline_last_stage()
+        cur_kwargs.update({
+            "pre_process": pre_process,
+            "post_process": post_process,
+        })
+        model = model_provider_func(*cur_args, **cur_kwargs)
+
+    if not isinstance(model, list):
+        model = [model]
+
+    # Set tensor model parallel attributes if not set.
+    # Only parameters that are already tensor model parallel have these
+    # attributes set for them. We should make sure the default attributes
+    # are set for all params so the optimizer can use them.
+    for model_module in model:
+        for param in model_module.parameters():
+            set_defaults_if_not_set_tensor_model_parallel_attributes(param)
+
+    # Print number of parameters.
+    if parallel_state.get_data_parallel_rank() == 0:
+        msg = " > number of parameters on (tensor, pipeline) model parallel rank ({}, {}): {}".format(
+            parallel_state.get_tensor_model_parallel_rank(),
+            parallel_state.get_pipeline_model_parallel_rank(),
+            sum([sum([p.nelement() for p in model_module.parameters()]) for model_module in model])
+        )
+        print(msg, flush=True)
+
+    # GPU allocation.
+    for model_module in model:
+        model_module.cuda(torch.cuda.current_device())
+
+    if wrap_with_ddp:
+        i = torch.cuda.current_device()
+        model = [
+            torch.nn.parallel.distributed.DistributedDataParallel(
+                model_module,
+                device_ids=[i],
+                output_device=i,
+                process_group=parallel_state.get_data_parallel_group(),
+            )
+            for model_module in model
+        ]
+    return model
+
+
+def _get_params_for_weight_decay_optimization(
+        model: Union[torch.nn.Module, List[torch.nn.Module]],
+) -> Dict[str, torch.nn.Parameter]:
+    """Divide params into with-weight-decay and without-weight-decay groups.
+    Layernorms and biases will have no weight decay but the rest will.
+    """
+    modules = listify_model(model)
+    from apex.normalization.fused_layer_norm import FusedLayerNorm  # NOQA
+    weight_decay_params = {'params': []}
+    no_weight_decay_params = {'params': [], 'weight_decay': 0.0}
+    for module in modules:
+        for module_ in module.modules():
+            if isinstance(module_, FusedLayerNorm):
+                no_weight_decay_params['params'].extend(
+                    [p for p in list(module_._parameters.values())
+                     if p is not None])
+            else:
+                weight_decay_params['params'].extend(
+                    [p for n, p in list(module_._parameters.items())
+                     if p is not None and n != 'bias'])
+                no_weight_decay_params['params'].extend(
+                    [p for n, p in list(module_._parameters.items())
+                     if p is not None and n == 'bias'])
+
+    return weight_decay_params, no_weight_decay_params
+
+
+def forward_step(
+        forward_step_func: FwdStepFunc,
+        batch: Batch,
+        model: torch.nn.Module,
+        input_tensor: Optional[torch.Tensor],
+        losses_reduced: List[torch.Tensor],
+):
+    """Forward step for passed-in model.
+
+    If first stage, input tensor is obtained from data_iterator, otherwise
+    passed-in input_tensor is used.
+
+    Returns output tensor.
+
+    Args:
+        forward_step_func: Model specific function. This takes a minibatch and model as its arguments and
+            returns the model's output and the loss function.
+        batch: minibatch
+        model: unwrappable model
+        input_tensor:
+        losses_reduced:
+
+    Returns:
+        output_tensor
+    """
+    # timers = get_timers()
+    # timers("forward-compute").start()
+    unwrapped_model = unwrap_model(model)
+    # NOTE (mkozuki): The passed `model` is expected to implement `set_input_tensor`.
+    # See https://github.com/NVIDIA/Megatron-LM/blob/5ac5571ba0265af4c491ee0af1508ca7589450c6/megatron/model/transformer.py#L679  # NOQA
+    # for the details of `set_input_tensor`.
+    unwrapped_model.set_input_tensor(input_tensor)
+    output_tensor, loss_func = forward_step_func(batch, model)
+    # print(f"forward_step| pipeline rank: {parallel_state.get_pipeline_model_parallel_rank()} is_pipeline_last_stage?: {parallel_state.is_pipeline_last_stage()}")
+    if parallel_state.is_pipeline_last_stage():
+        output_tensor = loss_func(output_tensor)
+        loss, loss_reduced = output_tensor
+        output_tensor = loss / get_num_microbatches()
+        losses_reduced.append(loss_reduced)
+    # timers("forward-compute").stop()
+
+    return output_tensor
+
+
+def backward_step(
+        input_tensor: Optional[torch.Tensor],
+        output_tensor: torch.Tensor,
+        output_tensor_grad: Optional[torch.Tensor],
+) -> Optional[torch.Tensor]:
+    """Backward step through passed-in output tensor.
+
+    If last stage, output_tensor_grad is None, otherwise gradient of loss
+    with respect to stage's output tensor.
+
+    Returns gradient of loss with respect to input tensor (None if first
+    stage).
+
+    Args:
+        input_tensor:
+        output_tensor:
+        output_tensor_grad:
+    Returns:
+        input_tensor_grad
+    """
+
+    # timers = get_timers()
+    # timers("backward-compute").start()
+    # Retain the grad on the input_tensor.
+
+    # if parallel_state.get_pipeline_model_parallel_rank() == 0:
+    #     print(f"{input_tensor}, {output_tensor}, {output_tensor_grad}")
+    if input_tensor is not None:
+        input_tensor.retain_grad()
+    # Backward pass.
+    # if output_tensor_grad is None:
+    #     output_tensor = optimizer.scale_loss(output_tensor)
+    torch.autograd.backward(output_tensor, grad_tensors=output_tensor_grad)
+    input_tensor_grad = None
+    if input_tensor is not None:
+        input_tensor_grad = input_tensor.grad
+    # timers("backward-compute").stop()
+
+    return input_tensor_grad

apex/transformer/pipeline_parallel/schedules/fwd_bwd_no_pipelining.py

+from contextlib import contextmanager
+from typing import List, Union
+
+import torch
+
+from apex.transformer.pipeline_parallel.utils import listify_model
+from apex.transformer.pipeline_parallel.utils import get_num_microbatches
+from apex.transformer.pipeline_parallel.utils import get_kth_microbatch
+from apex.transformer.pipeline_parallel.schedules.common import Batch, FwdStepFunc
+from apex.transformer.pipeline_parallel.schedules.common import forward_step
+from apex.transformer.pipeline_parallel.schedules.common import backward_step
+
+
+@contextmanager
+def placeholder_handler():
+    try:
+        yield
+    finally:
+        pass
+
+
+# TODO (mkozuki): Confirm this will be used or not.
+# TODO (mkozuki): Fix if necessary. Currently I'm seeing failure if `not forward_only` and
+#   the last `backward_step` seems to fail. However, note the possibility of my test script is wrong.
+def forward_backward_no_pipelining(
+        forward_step_func: FwdStepFunc,
+        batch: Batch,
+        model: Union[torch.nn.Module, List[torch.nn.Module]],
+        *,
+        forward_only: bool,
+        **kwargs,
+):
+    """Run forward and backward passes with no pipeline parallelism (no inter-stage communication).
+
+    This pipeline parallel scheduling handles the last microbatch differently to synchronize gradients.
+
+    Args:
+        forward_step_func: A function which takes a minibatch and model as its arguments and
+            returns model's forward output and the loss function.
+            The loss function is supposed to take one `torch.Tensor` and
+            return a `torch.Tensor` of loss and a dictionary of `str` and `torch.Tensor`.
+        batch: A List of torch.Tensors
+        model: A `torch.nn.Module` or a list of `torch.nn.Module`.
+
+    Keyword args:
+        forward_only:
+        **kwargs: Added to handle `tensor_shape` which has no effect on this function.
+
+    Returns:
+        a list of dictionaries of loss `torch.Tensor`s if the last stage, empty list otherwise.
+    """
+    model = listify_model(model)
+    if len(model) != 1:
+        msg = f"`model` is expected be a `nn.Module`, but {type(model)}"
+        raise RuntimeError(msg)
+    model = model[0]
+
+    context_handler = placeholder_handler
+    if isinstance(model, torch.nn.parallel.distributed.DistributedDataParallel):
+        context_handler = model.no_sync
+
+    losses_reduced = []
+    input_tensor, output_tensor_grad = None, None
+    num_micro_batches = get_num_microbatches()
+    with context_handler():
+        for i in range(num_micro_batches - 1):
+            cur_micro_batch = get_kth_microbatch(batch, i)
+            output_tensor = forward_step(
+                forward_step_func, cur_micro_batch, model, input_tensor, losses_reduced)
+            if not forward_only:
+                backward_step(input_tensor, output_tensor, output_tensor_grad)
+
+    # Run computation for last microbatch out of context handler (want to
+    # synchronize gradients).
+    output_tensor = forward_step(
+        forward_step_func, get_kth_microbatch(batch, num_micro_batches - 1), model, input_tensor, losses_reduced
+    )
+    if not forward_only:
+        backward_step(input_tensor, output_tensor, output_tensor_grad)
+
+    return losses_reduced

apex/transformer/pipeline_parallel/schedules/fwd_bwd_pipelining_with_interleaving.py

+from typing import List, Union, Optional
+
+import torch
+
+from apex.transformer import parallel_state
+from apex.transformer.pipeline_parallel import p2p_communication
+from apex.transformer.pipeline_parallel.schedules.common import Batch, FwdStepFunc
+from apex.transformer.pipeline_parallel.schedules.common import backward_step
+from apex.transformer.pipeline_parallel.schedules.common import forward_step
+from apex.transformer.pipeline_parallel.utils import get_kth_microbatch
+from apex.transformer.pipeline_parallel.utils import get_num_microbatches
+from apex.transformer.utils import rank_print
+
+
+# TODO (mkozuki): Reduce cyclomatic complexity
+def _forward_backward_pipelining_with_interleaving(
+        forward_step_func: FwdStepFunc,
+        batch: List[Batch],
+        model: List[torch.nn.Module],
+        *,
+        forward_only: bool,
+        tensor_shape: Optional[Union[List[int], torch.Size]] = None,
+):
+    """Run interleaved 1F1B schedule with communication between pipeline stages as needed.
+
+    This function assumes `batch` and `model` is a list of `Batch`'s and a list of `torch.nn.Module`, respectively.
+    This means that model is split into model chunks.
+
+    This pipeline parallel scheduling consists of three steps:
+        1. warmup
+        2. 1F1B a.k.a. steady state
+        3. cooldown
+    Note that if `forward_only` this scheduling consists of only warmup phase.
+
+    Args:
+        forward_step_func: A function which takes a minibatch and model as its arguments and
+            returns model's forward output and the loss function.
+            The loss function is supposed to take one `torch.Tensor` and
+            return a `torch.Tensor` of loss and a dictionary of `str` and `torch.Tensor`.
+        batch: A minibatch, i.e., a list of `torch.Tensor`'s.
+        model: A `torch.nn.Module` or a list of `torch.nn.Module`.
+
+    Keyword args:
+        forward_only:
+        tensor_shape: Shape of tensor.
+
+    Returns:
+        a list of loss `torch.Tensor`s if the last stage, empty list otherwise.
+    """
+    if not isinstance(model, list):
+        raise RuntimeError("`model` must be a list of `nn.Module`'s'")
+    # TODO (mkozuki): Sanity check the following condition.
+    if len(batch) != len(model):
+        msg = f"`batch` and `model` must have the same number of elements. Actual {len(batch)} and {len(model)}"
+        raise RuntimeError(msg)
+
+    num_model_chunks = len(model)
+    input_tensors = [[] for _ in range(num_model_chunks)]
+    output_tensors = [[] for _ in range(num_model_chunks)]
+    curr_iters = [0 for _ in range(num_model_chunks)]
+    losses_reduced = []
+    if not forward_only:
+        output_tensor_grads = [[] for _ in range(num_model_chunks)]
+
+    pipeline_parallel_size = parallel_state.get_pipeline_model_parallel_world_size()
+    pipeline_parallel_rank = parallel_state.get_pipeline_model_parallel_rank()
+
+    # Compute number of warmup and remaining microbatches.
+    num_microbatches = get_num_microbatches() * num_model_chunks
+    all_warmup_microbatches = False
+    if forward_only:
+        num_warmup_microbatches = num_microbatches
+    else:
+        # Run all forward passes and then all backward passes if number of
+        # microbatches is just the number of pipeline stages.
+        # Otherwise, perform (num_model_chunks-1)*pipeline_parallel_size on
+        # all workers, followed by more microbatches after depending on
+        # stage ID (more forward passes for earlier stages, later stages can
+        # immediately start with 1F1B).
+        if get_num_microbatches() == pipeline_parallel_size:
+            num_warmup_microbatches = num_microbatches
+            all_warmup_microbatches = True
+        else:
+            num_warmup_microbatches = (pipeline_parallel_size - pipeline_parallel_rank - 1) * 2
+            num_warmup_microbatches += (num_model_chunks - 1) * pipeline_parallel_size
+            num_warmup_microbatches = min(num_warmup_microbatches, num_microbatches)
+    num_microbatches_remaining = num_microbatches - num_warmup_microbatches
+
+
+    # TODO (mkozuki): Remove once debug gets done
+    # rank_print(
+    #     f"num_microbatches: {num_microbatches}, "
+    #     f"num_warmup_microbatches: {num_warmup_microbatches}, "
+    #     f"num_microbatches_remaining: {num_microbatches_remaining} -- "
+    # )
+
+    ###################################################################################################################
+    # Helper function definitions.
+    ###################################################################################################################
+    def get_model_chunk_id(microbatch_id: int, forward: bool) -> int:
+        """Helper function to get the model chunk ID given the iteration number."""
+        pipeline_parallel_size = parallel_state.get_pipeline_model_parallel_world_size()
+        microbatch_id_in_group = microbatch_id % (pipeline_parallel_size * num_model_chunks)
+        model_chunk_id = microbatch_id_in_group // pipeline_parallel_size
+        if not forward:
+            model_chunk_id = num_model_chunks - model_chunk_id - 1
+        return model_chunk_id
+
+    def forward_step_helper(microbatch_id, curr_iters):
+        """Helper method to run forward step with model split into chunks
+        (run set_virtual_pipeline_model_parallel_rank() before calling
+        forward_step())."""
+        model_chunk_id = get_model_chunk_id(microbatch_id, forward=True)
+        parallel_state.set_virtual_pipeline_model_parallel_rank(model_chunk_id)
+
+        # forward step
+        if (
+                parallel_state.is_pipeline_first_stage() and
+                len(input_tensors[model_chunk_id]) == len(output_tensors[model_chunk_id])
+        ):
+            input_tensors[model_chunk_id].append(None)
+        input_tensor = input_tensors[model_chunk_id][-1]
+        output_tensor = forward_step(
+            forward_step_func,
+            get_kth_microbatch(batch[model_chunk_id], curr_iters[model_chunk_id]),
+            model[model_chunk_id],
+            input_tensor,
+            losses_reduced,
+        )
+        curr_iters[model_chunk_id] += 1
+        output_tensors[model_chunk_id].append(output_tensor)
+
+        # if forward-only, no need to save tensors for a backward pass
+        if forward_only:
+            input_tensors[model_chunk_id].pop()
+            output_tensors[model_chunk_id].pop()
+
+        return output_tensor
+
+    def backward_step_helper(microbatch_id):
+        """Helper method to run backward step with model split into chunks
+        (run set_virtual_pipeline_model_parallel_rank() before calling
+        backward_step())."""
+        model_chunk_id = get_model_chunk_id(microbatch_id, forward=False)
+        parallel_state.set_virtual_pipeline_model_parallel_rank(model_chunk_id)
+
+        if parallel_state.is_pipeline_last_stage():
+            if len(output_tensor_grads[model_chunk_id]) == 0:
+                output_tensor_grads[model_chunk_id].append(None)
+        input_tensor = input_tensors[model_chunk_id].pop(0)
+        output_tensor = output_tensors[model_chunk_id].pop(0)
+        output_tensor_grad = output_tensor_grads[model_chunk_id].pop(0)
+        input_tensor_grad = backward_step(input_tensor, output_tensor, output_tensor_grad)
+
+        return input_tensor_grad
+
+    ###################################################################################################################
+    # Run warmup forward passes.
+    ###################################################################################################################
+    parallel_state.set_virtual_pipeline_model_parallel_rank(0)
+    input_tensors[0].append(p2p_communication.recv_forward(tensor_shape=tensor_shape))
+    for k in range(num_warmup_microbatches):
+        # rank_print(f"warmup iter: {k}")
+        output_tensor = forward_step_helper(k, curr_iters)
+
+        # Determine if tensor should be received from previous stage.
+        next_forward_model_chunk_id = get_model_chunk_id(k + 1, forward=True)
+        recv_prev = True
+        if parallel_state.is_pipeline_first_stage(ignore_virtual=True):
+            if next_forward_model_chunk_id == 0:
+                recv_prev = False
+        if k == (num_microbatches - 1):
+            recv_prev = False
+
+        # Don't send tensor downstream if on last stage.
+        if parallel_state.is_pipeline_last_stage():
+            output_tensor = None
+
+        # rank_print(f"recv_prev: {recv_prev}")
+        # Send and receive tensors as appropriate (send tensors computed
+        # in this iteration; receive tensors for next iteration).
+        if k == (num_warmup_microbatches - 1) and not forward_only and not all_warmup_microbatches:
+            input_tensor_grad = None
+            recv_next = True
+            # rank_print(f"recv_next: {recv_next}")
+            if parallel_state.is_pipeline_last_stage(ignore_virtual=True):
+                recv_next = False
+            (
+                input_tensor,
+                output_tensor_grad,
+            ) = p2p_communication.send_forward_backward_recv_forward_backward(
+                output_tensor,
+                input_tensor_grad,
+                recv_prev=recv_prev,
+                recv_next=recv_next,
+                tensor_shape=tensor_shape,
+            )
+            output_tensor_grads[num_model_chunks - 1].append(output_tensor_grad)
+        else:
+            # rank_print("send_forward_recv_forward start")
+            input_tensor = p2p_communication.send_forward_recv_forward(output_tensor, recv_prev=recv_prev, tensor_shape=tensor_shape)
+            # rank_print("send_forward_recv_forward finish")
+        # rank_print("communication done")
+        input_tensors[next_forward_model_chunk_id].append(input_tensor)
+
+    ###################################################################################################################
+    # Run 1F1B in steady state.
+    ###################################################################################################################
+    for k in range(num_microbatches_remaining):
+        # Forward pass.
+        forward_k = k + num_warmup_microbatches
+        output_tensor = forward_step_helper(forward_k, curr_iters)
+
+        # Backward pass.
+        backward_k = k
+        input_tensor_grad = backward_step_helper(backward_k)
+
+        # Send output_tensor and input_tensor_grad, receive input_tensor
+        # and output_tensor_grad.
+
+        # Determine if current stage has anything to send in either direction,
+        # otherwise set tensor to None.
+        forward_model_chunk_id = get_model_chunk_id(forward_k, forward=True)
+        parallel_state.set_virtual_pipeline_model_parallel_rank(forward_model_chunk_id)
+        if parallel_state.is_pipeline_last_stage():
+            output_tensor = None
+
+        backward_model_chunk_id = get_model_chunk_id(backward_k, forward=False)
+        parallel_state.set_virtual_pipeline_model_parallel_rank(backward_model_chunk_id)
+        if parallel_state.is_pipeline_first_stage():
+            input_tensor_grad = None
+
+        # Determine if peers are sending, and where in data structure to put
+        # received tensors.
+        recv_prev = True
+        if parallel_state.is_pipeline_first_stage(ignore_virtual=True):
+            # First stage is ahead of last stage by (pipeline_parallel_size - 1).
+            next_forward_model_chunk_id = get_model_chunk_id(
+                forward_k - (pipeline_parallel_size - 1), forward=True
+            )
+            if next_forward_model_chunk_id == (num_model_chunks - 1):
+                recv_prev = False
+            next_forward_model_chunk_id += 1
+        else:
+            next_forward_model_chunk_id = get_model_chunk_id(forward_k + 1, forward=True)
+
+        recv_next = True
+        if parallel_state.is_pipeline_last_stage(ignore_virtual=True):
+            # Last stage is ahead of first stage by (pipeline_parallel_size - 1).
+            next_backward_model_chunk_id = get_model_chunk_id(
+                backward_k - (pipeline_parallel_size - 1), forward=False
+            )
+            if next_backward_model_chunk_id == 0:
+                recv_next = False
+            next_backward_model_chunk_id -= 1
+        else:
+            next_backward_model_chunk_id = get_model_chunk_id(backward_k + 1, forward=False)
+
+        # If last iteration, don't receive; we already received one extra
+        # before the start of the for loop.
+        if k == (num_microbatches_remaining - 1):
+            recv_prev = False
+
+        # Communicate tensors.
+        (
+            input_tensor,
+            output_tensor_grad,
+        ) = p2p_communication.send_forward_backward_recv_forward_backward(
+            output_tensor,
+            input_tensor_grad,
+            recv_prev=recv_prev,
+            recv_next=recv_next,
+            tensor_shape=tensor_shape,
+        )
+
+        # Put input_tensor and output_tensor_grad in data structures in the
+        # right location.
+        if recv_prev:
+            input_tensors[next_forward_model_chunk_id].append(input_tensor)
+        if recv_next:
+            output_tensor_grads[next_backward_model_chunk_id].append(output_tensor_grad)
+
+    ###################################################################################################################
+    # Run cooldown backward passes (flush out pipeline).
+    ###################################################################################################################
+    if not forward_only:
+        if all_warmup_microbatches:
+            output_tensor_grads[num_model_chunks - 1].append(p2p_communication.recv_backward(tensor_shape=tensor_shape))
+        for k in range(num_microbatches_remaining, num_microbatches):
+            input_tensor_grad = backward_step_helper(k)
+            next_backward_model_chunk_id = get_model_chunk_id(k + 1, forward=False)
+            recv_next = True
+            if parallel_state.is_pipeline_last_stage(ignore_virtual=True):
+                if next_backward_model_chunk_id == (num_model_chunks - 1):
+                    recv_next = False
+            if k == (num_microbatches - 1):
+                recv_next = False
+            output_tensor_grads[next_backward_model_chunk_id].append(
+                p2p_communication.send_backward_recv_backward(input_tensor_grad, recv_next=recv_next, tensor_shape=tensor_shape)
+            )
+
+    return losses_reduced

apex/transformer/pipeline_parallel/schedules/fwd_bwd_pipelining_without_interleaving.py

+from typing import Union, List, Optional
+
+import torch
+
+from apex.transformer import parallel_state
+from apex.transformer.pipeline_parallel import p2p_communication
+from apex.transformer.pipeline_parallel.utils import get_kth_microbatch
+from apex.transformer.pipeline_parallel.utils import listify_model
+from apex.transformer.pipeline_parallel.utils import get_num_microbatches
+from apex.transformer.pipeline_parallel.schedules.common import Batch, FwdStepFunc
+from apex.transformer.pipeline_parallel.schedules.common import forward_step
+from apex.transformer.pipeline_parallel.schedules.common import backward_step
+from apex.transformer.utils import rank_print
+
+
+def forward_backward_pipelining_without_interleaving(
+        forward_step_func: FwdStepFunc,
+        batch: Batch,
+        model: Union[torch.nn.Module, List[torch.nn.Module]],
+        *,
+        forward_only: bool,
+        tensor_shape: Optional[Union[List[int], torch.Size]] = None,
+):
+    """Run non-interleaved 1F1B schedule, with communication between pipeline stages.
+
+    This pipeline parallel scheduling consists of three steps:
+        1. warmup
+        2. 1F1B a.k.a. steady state
+        3. cooldown if not forward_only
+
+    Args:
+        forward_step_func: A function which takes a minibatch and model as its arguments and
+            returns model's forward output and the loss function.
+            The loss function is supposed to take one `torch.Tensor` and
+            return a `torch.Tensor` of loss and a dictionary of `str` and `torch.Tensor`.
+        batch: A minibatch, i.e., a list of `torch.Tensor`'s.
+        model: A `torch.nn.Module` or a list of `torch.nn.Module`.
+
+    Keyword args:
+        forward_only:
+        tensor_shape: Shape of tensor. Required for P2P communication.
+
+    Returns:
+        a list of loss `torch.Tensor`s if the last stage, empty list otherwise.
+    """
+    # timers = get_timers()
+
+    model = listify_model(model)
+    if len(model) != 1:
+        msg = f"`model` is expected be a `nn.Module`, but {type(model)}"
+        raise RuntimeError(msg)
+    model = model[0]
+
+    # Compute number of warmup microbatches.
+    num_microbatches = get_num_microbatches()
+    num_warmup_microbatches = (
+        parallel_state.get_pipeline_model_parallel_world_size()
+        - parallel_state.get_pipeline_model_parallel_rank()
+        - 1
+    )
+    num_warmup_microbatches = min(num_warmup_microbatches, num_microbatches)
+    num_microbatches_remaining = num_microbatches - num_warmup_microbatches
+
+    # TODO (mkozuki): Remove once debug gets done
+    print(
+        f">>> rank: {torch.distributed.get_rank()}, "
+        f"num_microbatches: {num_microbatches}, "
+        f"num_warmup_microbatches: {num_warmup_microbatches}, "
+        f"num_microbatches_remaining: {num_microbatches_remaining} -- "
+    )
+
+    # Input, output tensors only need to be saved when doing backward passes
+    input_tensors = None
+    output_tensors = None
+    if not forward_only:
+        input_tensors = []
+        output_tensors = []
+    losses_reduced = []
+
+    ###################################################################################################################
+    # Run warmup forward passes.
+    ###################################################################################################################
+    # rank_print(f"warmup: {num_warmup_microbatches}")
+    for i in range(num_warmup_microbatches):
+        input_tensor = p2p_communication.recv_forward(tensor_shape=tensor_shape)
+        cur_microbatch = get_kth_microbatch(batch, i)
+        output_tensor = forward_step(forward_step_func, cur_microbatch, model, input_tensor, losses_reduced)
+        p2p_communication.send_forward(output_tensor, tensor_shape=tensor_shape)
+
+        if not forward_only:
+            input_tensors.append(input_tensor)
+            output_tensors.append(output_tensor)
+        # rank_print(f"warmup iter: {i + 1} / {num_warmup_microbatches}")
+    # rank_print("warmup done")
+
+    # Before running 1F1B, need to receive first forward tensor.
+    # If all microbatches are run in warmup / cooldown phase, then no need to
+    # receive this tensor here.
+    # rank_print(f"num microbatches remaining: {num_microbatches_remaining}")
+    if num_microbatches_remaining > 0:
+        # rank_print(f"recv_forward before steady state start")
+        input_tensor = p2p_communication.recv_forward(tensor_shape=tensor_shape)
+        # rank_print(f"recv_forward before steady state done")
+
+    ###################################################################################################################
+    # Run 1F1B in steady state.
+    ###################################################################################################################
+    # rank_print(f"steady: {num_microbatches_remaining} iters")
+    for i in range(num_microbatches_remaining):
+        # rank_print(f"steady: iter {i + 1} / {num_microbatches_remaining} iters")
+        # if not forward_only:
+        #     rank_print(f"len(input_tensors) = {len(input_tensors)}, len(output_tensors) = {len(output_tensors)}")
+        last_iteration = i == (num_microbatches_remaining - 1)
+
+        cur_microbatch = get_kth_microbatch(batch, i + num_warmup_microbatches)
+        output_tensor = forward_step(forward_step_func, cur_microbatch, model, input_tensor, losses_reduced)
+        if forward_only:
+            # rank_print(f"steady, no backward: `send_forward` start")
+            p2p_communication.send_forward(output_tensor, tensor_shape=tensor_shape)
+
+            if not last_iteration:
+                input_tensor = p2p_communication.recv_forward(tensor_shape=tensor_shape)
+            # rank_print(f"steady, no backward: `send_forward` finish")
+
+        else:
+            # rank_print("L.124 steady, backward: `send_forward_recv_backward` start")
+            output_tensor_grad = p2p_communication.send_forward_recv_backward(output_tensor, tensor_shape=tensor_shape)
+            # rank_print("L.124 steady, backward: `send_forward_recv_backward` finish")
+
+            # Add input_tensor and output_tensor to end of list.
+            input_tensors.append(input_tensor)
+            output_tensors.append(output_tensor)
+
+            # Pop input_tensor and output_tensor from the start of the list for the backward pass.
+            input_tensor = input_tensors.pop(0)
+            output_tensor = output_tensors.pop(0)
+
+            input_tensor_grad = backward_step(
+                input_tensor, output_tensor, output_tensor_grad
+            )
+
+            if last_iteration:
+                input_tensor = None
+                # rank_print(f"L.142 steady backward last iteration: `send_backward` start")
+                p2p_communication.send_backward(input_tensor_grad, tensor_shape=tensor_shape)
+                # rank_print(f"L.142 steady backward last iteration: `send_backward` finish")
+            else:
+                # rank_print(f"L.146 steady backward: `send_backward_recv_forward` start")
+                input_tensor = p2p_communication.send_backward_recv_forward(
+                    input_tensor_grad, tensor_shape=tensor_shape)
+                # rank_print(f"L.146 steady backward: `send_backward_recv_forward` finish")
+    # rank_print(f"steady: exit")
+    ###################################################################################################################
+    # Run cooldown backward passes.
+    ###################################################################################################################
+    if not forward_only:
+        # rank_print(f"cooldownk: {num_warmup_microbatches} iters")
+        for i in range(num_warmup_microbatches):
+            # rank_print(f"cooldown iter: {i + 1} / {num_warmup_microbatches}")
+            input_tensor = input_tensors.pop(0)
+            output_tensor = output_tensors.pop(0)
+
+            # rank_print(f"cooldown waiting for grad tensor")
+            output_tensor_grad = p2p_communication.recv_backward(tensor_shape=tensor_shape)
+
+            # rank_print(f"cooldown received grad tensor")
+            input_tensor_grad = backward_step(
+                input_tensor, output_tensor, output_tensor_grad
+            )
+
+            # rank_print(f"cooldown sending grad tensor")
+            p2p_communication.send_backward(input_tensor_grad, tensor_shape=tensor_shape)
+        # rank_print(f"cooldownk exit")
+
+    return losses_reduced

apex/transformer/pipeline_parallel/utils.py

+# coding=utf-8
+# Copyright (c) 2021, NVIDIA CORPORATION.  All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Utilities for pipeline model parallel."""
+from typing import Optional, List, Union
+
+import torch
+from torch.nn.parallel import DistributedDataParallel
+
+from apex.multi_tensor_apply import multi_tensor_applier
+import amp_C
+from apex.transformer import parallel_state
+from apex.transformer.microbatches import build_num_microbatches_calculator
+from apex.transformer.pipeline_parallel._timers import _Timers
+
+
+_GLOBAL_ARGS = None
+_GLOBAL_NUM_MICROBATCHES_CALCULATOR = None
+_GLOBAL_TOKENIZER = None
+_GLOBAL_TENSORBOARD_WRITER = None
+_GLOBAL_AUTORESUME = None
+_GLOBAL_TIMERS = None
+
+
+Shape = Union[List[int], torch.Size]
+
+
+def listify_model(model: Union[torch.nn.Module, List[torch.nn.Module]]) -> List[torch.nn.Module]:
+    if isinstance(model, list):
+        return model
+    return [model]
+
+
+def _ensure_var_is_initialized(var, name):
+    """Make sure the input variable is not None."""
+    assert var is not None, "{} is not initialized.".format(name)
+
+
+def _ensure_var_is_not_initialized(var, name):
+    """Make sure the input variable is not None."""
+    assert var is None, "{} is already initialized.".format(name)
+
+
+def setup_microbatch_calculator(
+        rank: int,
+        rampup_batch_size: Optional[List[int]],
+        global_batch_size: int,
+        micro_batch_size: int,
+        data_parallel_size: int,
+) -> None:
+    global _GLOBAL_NUM_MICROBATCHES_CALCULATOR
+    _ensure_var_is_not_initialized(_GLOBAL_NUM_MICROBATCHES_CALCULATOR, 'num microbatches calculator')
+
+    _GLOBAL_NUM_MICROBATCHES_CALCULATOR = build_num_microbatches_calculator(
+        rank, rampup_batch_size, global_batch_size, micro_batch_size, data_parallel_size)
+
+
+def get_micro_batch_size():
+    return _GLOBAL_NUM_MICROBATCHES_CALCULATOR.micro_batch_size
+
+
+def get_num_microbatches():
+    return _GLOBAL_NUM_MICROBATCHES_CALCULATOR.get()
+
+
+def get_current_global_batch_size():
+    return _GLOBAL_NUM_MICROBATCHES_CALCULATOR.get_current_global_batch_size()
+
+
+def update_num_microbatches(consumed_samples, consistency_check=True):
+    _GLOBAL_NUM_MICROBATCHES_CALCULATOR.update(consumed_samples, consistency_check)
+
+
+# note (mkozuki): Comment out in favor of `get_kth_microbatch`
+def _split_batch_into_microbatch(
+        batch: List[torch.Tensor],
+        *,
+        _micro_batch_size: Optional[int] = None,
+        _global_batch_size: Optional[int] = None,
+) -> List[List[torch.Tensor]]:
+    micro_batch_size = _micro_batch_size
+    global_batch_size = _global_batch_size
+    if micro_batch_size is None:
+        micro_batch_size = get_micro_batch_size()
+    if global_batch_size is None:
+        global_batch_size = get_current_global_batch_size()
+    for i in range(0, global_batch_size, micro_batch_size):
+        yield [x[i * micro_batch_size:(i + 1) * micro_batch_size] for x in batch]
+
+
+# TODO(mkozuki): Support non-tensor local minibatches?
+def get_kth_microbatch(batch: List[torch.Tensor], k: int) -> List[torch.Tensor]:
+    """Create a list of microbatches from a list of local minibatches.
+
+    This function creates a list of `k`th microbatches from a list of local minibatches.
+    `a local minibatch` consists of `global_batch_size / data_parallel_size` samples.
+    """
+    micro_batch_size = get_micro_batch_size()
+    return [x[k * micro_batch_size:(k + 1) * micro_batch_size] for x in batch]
+
+
+def get_autoresume():
+    return _GLOBAL_AUTORESUME
+
+
+def _set_timers():
+    """Initialize timers."""
+    global _GLOBAL_TIMERS
+    _ensure_var_is_not_initialized(_GLOBAL_TIMERS, "timers")
+    _GLOBAL_TIMERS = _Timers()
+
+
+def get_timers():
+    """Return timers."""
+    _ensure_var_is_initialized(_GLOBAL_TIMERS, "timers")
+    return _GLOBAL_TIMERS
+
+
+def print_rank_0(message: str) -> None:
+    """If distributed is initialized, print only on rank 0."""
+    if torch.distributed.is_initialized():
+        if torch.distributed.get_rank() == 0:
+            print(message, flush=True)
+    else:
+        print(message, flush=True)
+
+
+def is_last_rank():
+    return torch.distributed.get_rank() == (torch.distributed.get_world_size() - 1)
+
+
+def print_rank_last(message):
+    """If distributed is initialized, print only on last rank."""
+    if torch.distributed.is_initialized():
+        if is_last_rank():
+            print(message, flush=True)
+    else:
+        print(message, flush=True)
+
+
+def param_is_not_shared(param: torch.nn.Parameter) -> bool:
+    return getattr(param, "shared", False)
+
+
+def unwrap_model(model, module_instances=(DistributedDataParallel,)):
+    return_list = True
+    if not isinstance(model, list):
+        model = [model]
+        return_list = False
+    unwrapped_model = []
+    for model_module in model:
+        while isinstance(model_module, module_instances):
+            model_module = model_module.module
+        unwrapped_model.append(model_module)
+    if not return_list:
+        return unwrapped_model[0]
+    return unwrapped_model
+
+
+def calc_params_l2_norm(model: torch.nn.Module, bf16: bool):
+    """Calculate l2 norm of parameters """
+    # args = get_args()
+    if not isinstance(model, list):
+        model = [model]
+    # Remove duplicate params.
+    params_data = []
+    for model_ in model:
+        for param in model_.parameters():
+            is_not_shared = param_is_not_shared(param)
+            is_not_tp_duplicate = parallel_state.param_is_not_tensor_parallel_duplicate(param)
+            if is_not_shared and is_not_tp_duplicate:
+                if bf16:
+                    params_data.append(param.data.float())
+                else:
+                    params_data.append(param.data)
+    # Calculate norm
+    dummy_overflow_buf = torch.cuda.IntTensor([0])
+    norm, _ = multi_tensor_applier(
+        amp_C.multi_tensor_l2norm, dummy_overflow_buf, [params_data], False  # no per-parameter norm
+    )
+    norm_2 = norm * norm
+    # Sum across all model-parallel GPUs.
+    torch.distributed.all_reduce(
+        norm_2, op=torch.distributed.ReduceOp.SUM, group=parallel_state.get_model_parallel_group()
+    )
+    return norm_2.item() ** 0.5
+
+
+def average_losses_across_data_parallel_group(losses):
+    """Reduce a tensor of losses across all GPUs."""
+    averaged_losses = torch.cat([loss.clone().detach().view(1) for loss in losses])
+    torch.distributed.all_reduce(averaged_losses, group=parallel_state.get_data_parallel_group())
+    averaged_losses = averaged_losses / torch.distributed.get_world_size(
+        group=parallel_state.get_data_parallel_group()
+    )
+
+    return averaged_losses
+
+
+def report_memory(name):
+    """Simple GPU memory report."""
+    mega_bytes = 1024.0 * 1024.0
+    string = name + " memory (MB)"
+    string += " | allocated: {}".format(torch.cuda.memory_allocated() / mega_bytes)
+    string += " | max allocated: {}".format(torch.cuda.max_memory_allocated() / mega_bytes)
+    string += " | reserved: {}".format(torch.cuda.memory_reserved() / mega_bytes)
+    string += " | max reserved: {}".format(torch.cuda.max_memory_reserved() / mega_bytes)
+    if parallel_state.get_data_parallel_rank() == 0:
+        print("[Rank {}] {}".format(torch.distributed.get_rank(), string), flush=True)
+
+
+def print_params_min_max_norm(optimizer, iteration):
+    """Print min, max, and norm of all parameters."""
+    index = 0
+    rank = torch.distributed.get_rank()
+    string = "iteration, rank, index, tensor-model-parallel, min, max, norm\n"
+    optimizer_ = optimizer.optimizer
+    for param_group in optimizer_.param_groups:
+        for param in param_group["params"]:
+            index += 1
+            min_ = param.data.min()
+            max_ = param.data.max()
+            norm = torch.linalg.norm(param.data)
+            string += "{:7d}, {:4d}, {:4d}, {:2d}, ".format(
+                iteration, rank, index, int(param.tensor_model_parallel)
+            )
+            string += "{:.6E}, {:.6E}, {:.6E}\n".format(min_, max_, norm)
+    print(string, flush=True)
+
+
+# NOTE (mkozuki): APEX doesn't have anything equivalent for
+# `_GLOBAL_ADLR_AUTORESUME` like Megatron-LM.
+# def check_adlr_autoresume_termination(iteration, model, optimizer, lr_scheduler, save: bool):
+#     """Check for autoresume signal and exit if it is received."""
+#     from apex.ppu.checkpointing import save_checkpoint
+#
+#     autoresume = get_adlr_autoresume()
+#     # Add barrier to ensure consistency.
+#     torch.distributed.barrier()
+#     if autoresume.termination_requested():
+#         if save:
+#             save_checkpoint(iteration, model, optimizer, lr_scheduler)
+#         print_rank_0(">>> autoresume termination request found!")
+#         if torch.distributed.get_rank() == 0:
+#             autoresume.request_resume()
+#         print_rank_0(">>> training terminated. Returning")
+#         sys.exit(0)
+
+
+def get_ltor_masks_and_position_ids(
+    data, eod_token, reset_position_ids, reset_attention_mask, eod_mask_loss
+):
+    """Build masks and position id for left to right model."""
+
+    # Extract batch size and sequence length.
+    micro_batch_size, seq_length = data.size()
+
+    # Attention mask (lower triangular).
+    if reset_attention_mask:
+        att_mask_batch = micro_batch_size
+    else:
+        att_mask_batch = 1
+    attention_mask = torch.tril(
+        torch.ones((att_mask_batch, seq_length, seq_length), device=data.device)
+    ).view(att_mask_batch, 1, seq_length, seq_length)
+
+    # Loss mask.
+    loss_mask = torch.ones(data.size(), dtype=torch.float, device=data.device)
+    if eod_mask_loss:
+        loss_mask[data == eod_token] = 0.0
+
+    # Position ids.
+    position_ids = torch.arange(seq_length, dtype=torch.long, device=data.device)
+    position_ids = position_ids.unsqueeze(0).expand_as(data)
+    # We need to clone as the ids will be modifed based on batch index.
+    if reset_position_ids:
+        position_ids = position_ids.clone()
+
+    if reset_position_ids or reset_attention_mask:
+        # Loop through the batches:
+        for b in range(micro_batch_size):
+
+            # Find indecies where EOD token is.
+            eod_index = position_ids[b, data[b] == eod_token]
+            # Detach indecies from positions if going to modify positions.
+            if reset_position_ids:
+                eod_index = eod_index.clone()
+
+            # Loop through EOD indecies:
+            prev_index = 0
+            for j in range(eod_index.size()[0]):
+                i = eod_index[j]
+                # Mask attention loss.
+                if reset_attention_mask:
+                    attention_mask[b, 0, (i + 1) :, : (i + 1)] = 0
+                # Reset positions.
+                if reset_position_ids:
+                    position_ids[b, (i + 1) :] -= i + 1 - prev_index
+                    prev_index = i + 1
+
+    # Convert attention mask to binary:
+    attention_mask = attention_mask < 0.5
+
+    return attention_mask, loss_mask, position_ids

apex/transformer/tensor_parallel/__init__.py

 # coding=utf-8
-# Copyright (c) 2020, NVIDIA CORPORATION.  All rights reserved.
+# Copyright (c) 2021, NVIDIA CORPORATION.  All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -15,11 +15,11 @@
 
 """Model parallel utility interface."""
 
-from .cross_entropy import vocab_parallel_cross_entropy
+from apex.transformer.tensor_parallel.cross_entropy import vocab_parallel_cross_entropy
 
-from .data import broadcast_data
+from apex.transformer.tensor_parallel.data import broadcast_data
 
-from .layers import (
+from apex.transformer.tensor_parallel.layers import (
     ColumnParallelLinear,
     RowParallelLinear,
     VocabParallelEmbedding,
@@ -28,7 +28,7 @@ from .layers import (
     copy_tensor_model_parallel_attributes,
 )
 
-from .mappings import (
+from apex.transformer.tensor_parallel.mappings import (
     copy_to_tensor_model_parallel_region,
     gather_from_tensor_model_parallel_region,
     reduce_from_tensor_model_parallel_region,
@@ -41,11 +41,9 @@ from .random import (
     init_checkpointed_activations_memory_buffer,
     model_parallel_cuda_manual_seed,
     reset_checkpointed_activations_memory_buffer,
-    gather_split_1d_tensor,
-    split_tensor_into_1d_equal_chunks,
 )
 
-from .utils import divide, split_tensor_along_last_dim
+from apex.transformer.tensor_parallel.utils import split_tensor_along_last_dim
 
 
 __all__ = [
@@ -71,9 +69,6 @@ __all__ = [
     "init_checkpointed_activations_memory_buffer",
     "model_parallel_cuda_manual_seed",
     "reset_checkpointed_activations_memory_buffer",
-    "gather_split_1d_tensor",
-    "split_tensor_into_1d_equal_chunks",
     # utils.py
-    "divide",
     "split_tensor_along_last_dim",
 ]

apex/transformer/tensor_parallel/cross_entropy.py

 # coding=utf-8
-# Copyright (c) 2020, NVIDIA CORPORATION.  All rights reserved.
+# Copyright (c) 2021, NVIDIA CORPORATION.  All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -14,10 +14,10 @@
 # limitations under the License.
 import torch
 
-from ..parallel_state import get_tensor_model_parallel_group
-from ..parallel_state import get_tensor_model_parallel_rank
-from ..parallel_state import get_tensor_model_parallel_world_size
-from .utils import VocabUtility
+from apex.transformer.parallel_state import get_tensor_model_parallel_group
+from apex.transformer.parallel_state import get_tensor_model_parallel_rank
+from apex.transformer.parallel_state import get_tensor_model_parallel_world_size
+from apex.transformer.tensor_parallel.utils import VocabUtility
 
 
 class _VocabParallelCrossEntropy(torch.autograd.Function):