test_fsdp.py

# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
#
# This source code is licensed under the BSD license found in the
# LICENSE file in the root directory of this source tree.

import functools
import itertools
from math import inf
import pickle
import sys
from typing import Dict
import unittest
from unittest import mock

from parameterized import parameterized
import pytest
import torch
from torch import nn
import torch.distributed

from fairscale.nn.checkpoint.checkpoint_activations import checkpoint_wrapper
from fairscale.nn.data_parallel import FullyShardedDataParallel, TrainingState
from fairscale.utils import torch_version
from fairscale.utils.testing import (
    DeviceAndTypeCheckModule,
    DummyProcessGroup,
    dist_init,
    get_cycles_per_ms,
    objects_are_equal,
    spawn_for_all_world_sizes,
)

if torch_version() >= (1, 8, 0):
    from fairscale.optim.grad_scaler import ShardedGradScaler

# How to use remote-pdb: https://gist.github.com/sshleifer/9d43351957179c13606e015b072927d4
# All helper functions called by spawn must be either @classmethod, @staticmethod


class DistributedTest(unittest.TestCase):
    def setUp(self):
        if torch_version() < (1, 6, 0):
            raise unittest.SkipTest("Need pytorch version >= 1.6 due to lack of reduce_scatter")
        if not torch.cuda.is_available():
            raise unittest.SkipTest("CUDA not available, skipping test")
        if sys.platform == "win32":
            raise unittest.SkipTest("NCCL doesn't support Windows, skipping test")
        if torch.cuda.device_count() < 2:
            raise unittest.SkipTest("distributed tests require 2+ GPUs, skipping")

    @staticmethod
    def _train_for_several_steps(model, num_steps, autocast, lr=0.01, norm_type=None):
        model_device = next(model.parameters()).device
        # use SGD with momentum instead of Adam, since Adam is scale invariant
        # and this makes it bad for tests

        optim = torch.optim.SGD(params=model.parameters(), lr=lr, momentum=0.9)
        scaler = ShardedGradScaler()
        for _ in range(num_steps):
            optim.zero_grad()
            with torch.cuda.amp.autocast(enabled=autocast):
                # Inputs always cuda regardless of move_grads_cpu, or model.device
                input = model.module.get_input(torch.device("cuda"))
                output = model(*input)
                loss = model.module.get_loss(input, output).to(model_device)
            loss = scaler.scale(loss)
            assert loss.dtype == torch.float32
            model.module.run_backward(loss)
            if norm_type is not None:
                clip_norm = 0.3
                if isinstance(model, FullyShardedDataParallel):
                    model.clip_grad_norm_(clip_norm, norm_type)
                else:
                    torch.nn.utils.clip_grad_norm_(model.parameters(), clip_norm, norm_type)
            scaler.step(optim)
            scaler.update()
        if hasattr(model, "assert_idle"):
            model.assert_idle()
        if isinstance(model, FullyShardedDataParallel):
            model.assert_state(TrainingState.IDLE)
        return loss.detach()

    @staticmethod
    def get_wrapped_model(group, cuda_first=False, config={}, **model_kwargs) -> FullyShardedDataParallel:
        if cuda_first:
            model = FullyShardedDataParallel(TransformerWithSharedParams(group, **model_kwargs).cuda(), group, **config)
        else:
            model = FullyShardedDataParallel(TransformerWithSharedParams(group, **model_kwargs), group, **config).cuda()
        return model

    @classmethod
    def _test_identical_outputs(
        cls,
        model_init_fn,
        config,
        rank,
        group,
        num_steps=2,
        use_cuda=True,
        lr=0.01,
        ref_ddp_fn=None,
        norm_type=2,
    ):
        if config.get("mixed_precision", False):
            autocast = True
            # Force the compute dtype to be torch.float32 so that we get
            # identical results as PyTorch DDP when using autocast. Note that
            # this will cause the all-gather to happen in FP32, which is slower
            # than necessary in most cases.
            config["compute_dtype"] = torch.float32
        else:
            autocast = False

        # Establish reference behavior with PyTorch DDP (+ optionally autocast).
        model = model_init_fn(group=group, wrapper_config=None).cuda()
        if ref_ddp_fn is None:
            model = nn.parallel.DistributedDataParallel(
                model, device_ids=[rank], output_device=rank, process_group=group
            )
        else:
            model = ref_ddp_fn(model, group)
        ref_loss = cls._train_for_several_steps(model, num_steps, autocast, lr=lr, norm_type=norm_type)
        ref_state_dict = model.module.state_dict()
        if config.get("cpu_offload", False):
            for k in ref_state_dict.keys():
                ref_state_dict[k] = ref_state_dict[k].cpu()

        # Confirm we get the same behavior using FullyShardedDataParallel.
        model = FullyShardedDataParallel(model_init_fn(group=group, wrapper_config=config), group, **config)
        if use_cuda:
            model = model.cuda()
        else:
            assert next(model.parameters()).device == torch.device("cpu")
        shard_loss = cls._train_for_several_steps(model, num_steps, autocast, lr=lr, norm_type=norm_type)
        if config.get("cpu_offload", False):
            # In pytorch 1.10, assert_allclose below checks for tensor device match. Therefore,
            # we need to move the CPU tensor to CUDA in case we are doing cpu_offload.
            shard_loss = shard_loss.cuda()
        shard_state_dict = model.state_dict()

        try:
            torch.testing.assert_allclose(ref_loss, shard_loss)
            assert objects_are_equal(ref_state_dict, shard_state_dict, raise_exception=True)
        except (AssertionError, RuntimeError) as e:
            raise Exception(f"FullyShardedDataParallel didn't match PyTorch DDP using config: {config}\n\n {e}")
        if config.get("flatten_parameters", True):
            metadata = model.local_metadata_dict()
            assert isinstance(metadata, dict)


class TestMixedPrecision(DistributedTest):
    def test_all_fp32(self):
        self._spawn_test_case(
            {"mixed_precision": False},
            False,  # autocast enabled
            torch.float32,  # expected_input_dtype
            torch.float32,  # expected_param_dtype
            torch.float32,  # expected_loss_dtype
            torch.float32,  # expected_reduce_dtype
        )

    def test_mixed_precision(self):
        self._spawn_test_case(
            {"mixed_precision": True},
            False,  # autocast enabled
            torch.float16,  # expected_input_dtype
            torch.float16,  # expected_param_dtype
            torch.float16,  # expected_loss_dtype
            torch.float16,  # expected_reduce_dtype
        )

    def test_mixed_precision_autocast(self):
        """If autocast enabled, loss should be fp32."""
        self._spawn_test_case(
            {"mixed_precision": True},
            True,  # autocast enabled
            torch.float16,  # expected_input_dtype
            torch.float16,  # expected_param_dtype
            torch.float32,  # expected_loss_dtype
            torch.float16,  # expected_reduce_dtype
        )

    def test_mixed_precision_autocast_buffer_type_fp32(self):
        """If autocast enabled, loss should be fp32."""
        self._spawn_test_case(
            {"mixed_precision": True, "buffer_dtype": torch.float32},
            True,  # autocast enabled
            torch.float16,  # expected_input_dtype
            torch.float16,  # expected_param_dtype
            torch.float32,  # expected_loss_dtype
            torch.float16,  # expected_reduce_dtype
            expected_buffer_type=torch.float32,
        )

    def test_mixed_precision_autocast_fp32_compute(self):
        self._spawn_test_case(
            {"mixed_precision": True, "compute_dtype": torch.float32},
            True,  # autocast enabled
            torch.float16,  # expected_input_dtype
            torch.float32,  # expected_param_dtype
            torch.float32,  # expected_loss_dtype
            torch.float32,  # expected_reduce_dtype
            expected_buffer_type=torch.float32,
        )

    def test_fp32_reduce_scatter(self):
        self._spawn_test_case(
            {"mixed_precision": True, "fp32_reduce_scatter": True},
            False,  # autocast enabled
            torch.float16,  # expected_input_dtype
            torch.float16,  # expected_param_dtype
            torch.float16,  # expected_loss_dtype
            torch.float32,  # expected_reduce_dtype
            expected_buffer_type=torch.float16,
        )

    def test_fp32_reduce_scatter_autocast(self):
        self._spawn_test_case(
            {"mixed_precision": True, "fp32_reduce_scatter": True},
            True,  # autocast enabled
            torch.float16,  # expected_input_dtype
            torch.float16,  # expected_param_dtype
            torch.float32,  # expected_loss_dtype
            torch.float32,  # expected_reduce_dtype
        )

    def _spawn_test_case(
        self,
        cfg,
        autocast_enabled,
        in_dtype,
        p_dtype,
        loss_dtype,
        reduce_dtype,
        expected_buffer_type=None,
        world_size=2,
    ):
        """Call test_dtypes inside of torch.multiprocessing.spawn"""
        fn = functools.partial(
            self._test_dtypes,
            cfg,
            autocast_enabled,
            in_dtype,
            p_dtype,
            loss_dtype,
            reduce_dtype,
            expected_buffer_type=expected_buffer_type,
        )
        spawn_and_init(fn, world_sizes=[world_size])

    @staticmethod
    def _test_dtypes(
        cfg: Dict, autocast, in_dtype, p_dtype, loss_dtype, reduce_dtype, rank, group, expected_buffer_type=None
    ):
        # Patch torch.distributed.reduce_scatter to check the dtype of the reduction
        orig_reduce_scatter = torch.distributed.reduce_scatter

        model: nn.Module = DeviceAndTypeCheckModule(
            expected_input_dtype=in_dtype,
            expected_param_dtype=p_dtype,
            expected_loss_dtype=loss_dtype,
            expected_buffer_dtype=expected_buffer_type,
        )

        def _reduce_scatter(output, input_list, **kwargs):
            for tensor in input_list:
                model._check("reduce_scatter.dtype", tensor.dtype, expected=reduce_dtype)
            return orig_reduce_scatter(output, input_list, **kwargs)

        with mock.patch("torch.distributed.reduce_scatter", new=_reduce_scatter):
            model = FullyShardedDataParallel(model, group, **cfg).cuda()
            device = next(model.parameters()).device
            x = torch.rand(2, 5).to(device)
            with torch.cuda.amp.autocast(enabled=autocast):
                loss = model(x)
            loss.backward()


keys = ["reshard_after_forward", "mixed_precision", "flatten_parameters"]
CONFIG_OPTIONS = [[dict(zip(keys, config))] for config in itertools.product([True, False], repeat=len(keys))]


def rename_test(testcase_func, param_num, param):
    return "%s_%s" % (
        testcase_func.__name__,
        parameterized.to_safe_name(str(param.args)),
    )


class TestComparisonToPyTorchDDP(DistributedTest):
    """
    Compare losses and parameter values after several updates when using
    PyTorch DDP vs. FullyShardedDataParallel.
    """

    @parameterized.expand(CONFIG_OPTIONS, name_func=rename_test)
    def test_nested_wrapped_model(self, config):
        test_fn = functools.partial(self._test_identical_outputs, NestedWrappedModule, config)
        spawn_and_init(test_fn)

    @parameterized.expand(CONFIG_OPTIONS, name_func=rename_test)
    def test_nested_all_wrapped_model(self, config):
        model_fn = functools.partial(NestedWrappedModule, wrap_everything=True)
        test_fn = functools.partial(self._test_identical_outputs, model_fn, config)
        spawn_and_init(test_fn)

    @parameterized.expand(CONFIG_OPTIONS, name_func=rename_test)
    def test_nested_all_wrapped_model_checkpoint(self, config):
        model_fn = functools.partial(NestedWrappedModule, wrap_everything=True, checkpoint=True)
        test_fn = functools.partial(self._test_identical_outputs, model_fn, config)
        spawn_and_init(test_fn)

    @parameterized.expand(CONFIG_OPTIONS, name_func=rename_test)
    def test_transformer_parameterized(self, config):
        # Test every combination of these options:
        spawn_and_init(functools.partial(self._test_identical_outputs, TransformerWithSharedParams, config))

    # testing moving params to cpu while using full and mixed precision
    @parameterized.expand([(True,), (False,)], name_func=rename_test)
    def test_cpu_offload_and_cpu_grads(self, mixed_precision):
        config = {"mixed_precision": mixed_precision, "cpu_offload": True}
        test_fn = functools.partial(
            self._test_identical_outputs, TransformerWithSharedParams, config, use_cuda=False, lr=0.01
        )
        spawn_and_init(test_fn)

    # testing full and mixed precision on the gpu
    @parameterized.expand([(True,), (False,)], name_func=rename_test)
    def test_no_cpu_offload_with_sharded_grad_scaler(self, mixed_precision):
        config = {"mixed_precision": mixed_precision, "move_params_to_cpu": False}
        test_fn = functools.partial(
            self._test_identical_outputs, TransformerWithSharedParams, config, use_cuda=True, lr=0.01
        )
        spawn_and_init(test_fn)

    def test_cpu_offload_and_cuda_grads_breaks(self):
        # If grads are on gpu, but model and optimizer are on cpu, backward breaks.
        config = {"mixed_precision": True, "cpu_offload": True, "move_grads_to_cpu": False}
        with self.assertRaises(Exception):  # RuntimeError inside spawn
            test_fn = functools.partial(
                self._test_identical_outputs, TransformerWithSharedParams, config, use_cuda=False
            )
            spawn_and_init(test_fn)

    def test_delayed_optim_step(self):
        # We use a model with a long CUDA delay right before the optimizer step.
        # This tests our streams logic, and that we don't start the FP32 -> FP16
        # transfer until after the optimization step completes.
        config = {"mixed_precision": True}
        model_fn = functools.partial(NestedWrappedModuleWithDelay, delay_after_loss_ms=250)
        test_fn = functools.partial(self._test_identical_outputs, model_fn, config)
        spawn_and_init(test_fn)

    def test_delayed_reduce_scatter(self):
        # We insert a delay in the torch.distributed.reduce_scatter op, so that
        # the post_backward_stream takes much longer than the backward pass.
        # This tests that we properly block at the end of the backward pass for
        # the reductions to finish.
        config = {"mixed_precision": True}
        model_fn = functools.partial(NestedWrappedModuleWithDelay, delay_before_reduction_ms=250)
        test_fn = functools.partial(self._test_identical_outputs, model_fn, config)
        spawn_and_init(test_fn)

    @parameterized.expand([[{"checkpoint_act": False}], [{"checkpoint_act": True}]], name_func=rename_test)
    def test_mixture_of_experts(self, moe_config):
        fsdp_config = {"mixed_precision": True}
        test_fn = functools.partial(
            self._test_identical_outputs,
            functools.partial(MixtureOfExperts, **moe_config),
            fsdp_config,
            # MixtureOfExperts implements custom reduce logic, so the reference
            # behavior should use that logic instead of PyTorch DDP.
            ref_ddp_fn=self._dummy_ddp_fn,
            norm_type=None,
        )
        spawn_and_init(test_fn)

    @parameterized.expand([[{"checkpoint_act": False}], [{"checkpoint_act": True}]], name_func=rename_test)
    def test_mixture_of_experts_with_delay_before_free(self, moe_config):
        fsdp_config = {"mixed_precision": True}
        test_fn = functools.partial(
            self._test_identical_outputs,
            functools.partial(MixtureOfExperts, delay_before_free_ms=250, **moe_config),
            fsdp_config,
            # MixtureOfExperts implements custom reduce logic, so the reference
            # behavior should use that logic instead of PyTorch DDP.
            ref_ddp_fn=self._dummy_ddp_fn,
            norm_type=None,
        )
        spawn_and_init(test_fn)

    def test_mixture_of_experts_grad_clip_breaks(self):
        config = {"mixed_precision": True}
        test_fn = functools.partial(
            self._test_identical_outputs,
            MixtureOfExperts,
            config,
            ref_ddp_fn=self._dummy_ddp_fn,
            norm_type=2,
        )
        with self.assertRaises(Exception):
            spawn_and_init(test_fn)

    @classmethod
    def _dummy_ddp_fn(self, model, group):
        return DummyDDP(model)

    @parameterized.expand([[1], [inf]], name_func=rename_test)
    def test_clip_norm_transformer(self, norm_type):
        config = {"mixed_precision": True}
        test_fn = functools.partial(
            self._test_identical_outputs,
            TransformerWithSharedParams,
            config,
            norm_type=norm_type,
        )
        spawn_and_init(test_fn)


class TestParamInit(DistributedTest):
    def test_param_change_after_init(self):
        test_fn = functools.partial(self._test_param_change_after_init, config={"mixed_precision": True})
        spawn_and_init(test_fn)

    @classmethod
    def _test_param_change_after_init(self, rank, group, config):
        # Establish reference behavior.
        model = self.get_wrapped_model(group, cuda_first=False, config=config)
        model.eval()  # no dropout for this test
        input = model.module.get_input(torch.device("cuda"))
        ref_output = model(*input)

        # Change the weights in place.
        model = self.get_wrapped_model(group, cuda_first=False, config=config)
        model.eval()  # no dropout for this test
        first_param = next(model.parameters())
        nn.init.normal_(first_param.data)
        new_output = model(*input)

        assert not objects_are_equal(ref_output, new_output), "new_output did not reflect change to param after init"


class TestSerialization(DistributedTest):
    @parameterized.expand([[False, False], [True, False], [True, True], [False, True]], name_func=rename_test)
    def test_pickle(self, mixed_precision, cpu_offload):
        """Ensure that wrapped modules can be pickled/unpickled."""
        config = {"mixed_precision": mixed_precision, "cpu_offload": cpu_offload}
        test_fn = functools.partial(self._test_pickle, config=config)
        spawn_and_init(test_fn, world_sizes=[2])

    @parameterized.expand([[False, False], [True, False], [True, True], [False, True]], name_func=rename_test)
    def test_multiprocessing(self, mixed_precision, cpu_offload):
        """Ensure that wrapped modules can be sent via multiprocessing."""
        config = {"mixed_precision": mixed_precision, "cpu_offload": cpu_offload}
        test_fn = functools.partial(self._test_multiprocessing, config=config)
        spawn_and_init(test_fn, world_sizes=[2])

    @classmethod
    def _test_pickle(self, rank, group, config):
        model = self._get_model(group, config)
        model = pickle.loads(pickle.dumps(model))
        if not config["cpu_offload"]:
            model = model.cuda()
        self._one_step(model, group)

    @classmethod
    def _test_multiprocessing(self, rank, group, config):
        mp = torch.multiprocessing.Pool(1)
        dummy_group = DummyProcessGroup(rank=group.rank(), size=group.size())
        model = mp.apply(self._get_model, (dummy_group, config))
        if not config["cpu_offload"]:
            model = model.cuda()
        self._one_step(model, group)

    @classmethod
    def _get_model(self, group, config):
        with torch.no_grad():  # required for multiprocessing
            model = NestedWrappedModule(group, wrapper_config=config)
            return FullyShardedDataParallel(model, group, **config)

    @classmethod
    def _one_step(self, model, group):
        # reset the process group (required after unpickling)
        for m in model.modules():
            if isinstance(m, FullyShardedDataParallel):
                m.process_group = group
        optim = torch.optim.SGD(model.parameters(), lr=0.01, momentum=0.9)
        input = model.module.get_input(torch.device("cuda"))
        output = model(*input)
        loss = model.module.get_loss(input, output)
        model.module.run_backward(loss)
        optim.step()


@pytest.mark.skipif(torch_version() < (1, 8, 0), reason="pytorch version >= 1.8.0 required")
class TestHooks(DistributedTest):
    # Feel free to modify these tests as the implementation changes.
    # They aspire to make sure that backward hooks are registered and used
    @parameterized.expand([[True], [False]])
    def test_output_backward_hooks(self, cuda_first):
        fn = functools.partial(self._test_output_backward_hooks, cuda_first=cuda_first)
        spawn_and_init(fn)

    def test_backward_hooks_after_save(self):
        fn = functools.partial(self._test_backward_hooks_after_save, cuda_first=False)
        spawn_and_init(fn)

    @classmethod
    def _test_backward_hooks_after_save(self, rank, group, cuda_first=False):
        model = self.get_wrapped_model(group, cuda_first=cuda_first)
        self._train_for_several_steps(model, 2, model.mixed_precision)
        state_1 = model.local_state_dict()
        model.load_local_state_dict(state_1)
        self._test_output_backward_hooks(rank, group, cuda_first=cuda_first, model=model)

    @classmethod
    def _test_output_backward_hooks(self, rank, group, cuda_first=False, model=None):
        if model is None:
            model = self.get_wrapped_model(group, cuda_first=cuda_first)
        optim = torch.optim.SGD(model.parameters(), lr=0.01, momentum=0.9)
        optim.zero_grad()
        # Inputs always cuda regardless of move_grads_cpu, or model.device
        input = model.module.get_input(torch.device("cuda"))
        output = model(*input)
        assert len(output._backward_hooks) == 1  # this is pre-bwd hook
        loss = model.module.get_loss(input, output).cuda()
        loss.backward()
        assert len(output._backward_hooks) == 1  # It doesn't get removed
        optim.step()
        assert len(output._backward_hooks) == 1

    @parameterized.expand([[True], [False]])
    def test_register_functions_called(self, cuda_first):
        fn = functools.partial(self._test_register_functions_called, cuda_first=cuda_first)
        spawn_and_init(fn)

    @classmethod
    def _test_register_functions_called(self, rank, group, cuda_first=False):
        """Tests that _register_{pre|post}_backward_hooks called during forward."""
        model = self.get_wrapped_model(group, cuda_first=cuda_first)
        input = model.module.get_input(torch.device("cuda"))
        model._register_post_backward_hooks = mock.MagicMock(return_value=None)
        model._register_pre_backward_hooks = mock.MagicMock(return_value=None)
        assert not model._register_post_backward_hooks.called
        assert not model._register_pre_backward_hooks.called
        model(*input)
        assert model._register_post_backward_hooks.called
        assert model._register_pre_backward_hooks.called


@pytest.mark.skipif(torch_version() < (1, 8, 0), reason="pytorch version >= 1.8.0 required")
class TestNoGrad(DistributedTest):
    @parameterized.expand(CONFIG_OPTIONS, name_func=rename_test)
    def test_transformer_parameterized(self, config):
        test_fn = functools.partial(self._test_transformer, config=config)
        spawn_and_init(test_fn)

    @classmethod
    def _test_transformer(self, rank, group, config):
        autocast = config["mixed_precision"]

        # Train model for a step
        model = self.get_wrapped_model(group, cuda_first=False, config=config)
        self._train_for_several_steps(model, 1, autocast)

        model.eval()  # no dropout for this test

        # Eval in standard mode (i.e., without no_grad)
        input = model.module.get_input(torch.device("cuda"))
        ref_output = model(*input)

        # Eval with no_grad and compare
        with torch.no_grad():
            no_grad_output = model(*input)

        assert objects_are_equal(ref_output, no_grad_output, raise_exception=True)


@pytest.mark.skipif(torch_version() < (1, 8, 0), reason="pytorch version >= 1.8.0 required")
class TestModuleProperties(DistributedTest):
    @parameterized.expand([[{"flatten_parameters": False}], [{"flatten_parameters": True}]], name_func=rename_test)
    def test_named_parameters(self, config):
        test_fn = functools.partial(self._test_named_params, config=config)
        spawn_and_init(test_fn)

    @classmethod
    def _test_named_params(self, rank, group, config):
        # Get the named parameters before wrapping.
        before_wrap_model = TransformerWithSharedParams(group)
        before_wrap_params = before_wrap_model.named_parameters()

        # Train the model for 1 step.
        model = self.get_wrapped_model(group, cuda_first=False, config=config)
        self._train_for_several_steps(model, 1, autocast=False)

        # Get the named parameters after wrapping to compare.
        after_wrap_params = model.named_parameters()

        if not config["flatten_parameters"]:
            for before_nm, after_nm in zip(before_wrap_params, after_wrap_params):
                assert before_nm[0] == after_nm[0]
        else:
            named_params_flat = [p for p in after_wrap_params][0][0]
            assert "flat_param_0" in named_params_flat

        # Compare name and size under the `summon_full_params` context.
        with model.summon_full_params():
            after_wrap_params = model.named_parameters()

            for before_nm, after_nm_original in zip(before_wrap_params, after_wrap_params):
                assert before_nm[0] == after_nm_original[0]
                torch.testing.assert_allclose(before_nm[1].shape, after_nm_original[1].cpu().shape)


class TransformerWithSharedParams(nn.Module):
    def __init__(self, group, *unused_args, d_vocab=23, d_model=16, add_bn=True, **unused_kwargs):
        super().__init__()
        self.rank = group.rank()
        self.world_size = group.size()
        torch.manual_seed(0)  # keep everything deterministic
        assert d_vocab >= 12  # we use torch.arange(12) as input
        self.embed_tokens = nn.Embedding(d_vocab, d_model)
        self.transformer = nn.Transformer(
            d_model=d_model,
            num_encoder_layers=2,
            num_decoder_layers=2,
            dim_feedforward=8,
            dropout=0.1,
        )
        self.output_proj = nn.Linear(d_model, d_vocab)

        # share the embedding and output projection weights
        self.output_proj.weight = self.embed_tokens.weight
        self.register_buffer("vocab_bias", self.embed_tokens.weight.new_ones((d_model,)))
        self.register_buffer("long_buffer", torch.zeros_like(self.vocab_bias, dtype=torch.long))

        self.bs = 2
        self.bn = torch.nn.BatchNorm1d(self.bs) if add_bn else torch.nn.Identity()

    def get_input(self, device):
        torch.manual_seed(1 + self.rank)  # keep everything deterministic
        src = torch.arange(12, device=device).view(6, self.bs)  # T x B
        tgt = torch.arange(self.bs * 4, device=device).view(4, self.bs)  # T x B
        return (src, tgt)

    def forward(self, src_ids, tgt_ids):
        src = self.embed_tokens(src_ids)
        src = src + self.vocab_bias + self.long_buffer.type_as(src)
        tgt = self.embed_tokens(tgt_ids)
        tgt = self.bn(tgt)
        x = self.transformer(src, tgt)
        return self.output_proj(x)

    def get_loss(self, input, output):
        _, tgt = input
        return nn.functional.cross_entropy(output.view(-1, output.size(-1)), tgt.view(-1), reduction="sum")

    def run_backward(self, loss):
        loss.backward()


class NestedWrappedModule(nn.Module):
    def __init__(self, group, wrapper_config, wrap_everything=False, checkpoint=False):
        super().__init__()
        self.rank = group.rank()
        self.world_size = group.size()
        self.wrapper_config = wrapper_config

        def _maybe_wrap(layer):
            if wrapper_config is not None:
                return FullyShardedDataParallel(layer, group, **wrapper_config)
            return layer

        torch.manual_seed(0)  # keep everything deterministic
        self.module = nn.Sequential(
            nn.Linear(8, 4),
            _maybe_wrap(
                nn.Sequential(
                    _maybe_wrap(nn.Linear(4, 16)),
                    nn.Linear(16, 16),
                )
            ),
            _maybe_wrap(nn.Linear(16, 4)),
            nn.Linear(4, 8),
        )

        # Wrap all modules triggers a corner case where root FSDP doesn't have any params.
        # Test it with checkpoint_wrapper as well to validate final backward callback
        # is queued correctly when root FSDP does not have any params and every layer is
        # wrapped as FSDP(checkpoint(module)).
        if wrap_everything:
            if checkpoint:
                self.module = nn.Sequential(
                    _maybe_wrap(checkpoint_wrapper(nn.Linear(8, 4))),
                    _maybe_wrap(checkpoint_wrapper(nn.Linear(4, 16))),
                    _maybe_wrap(checkpoint_wrapper(nn.Linear(16, 4))),
                    _maybe_wrap(checkpoint_wrapper(nn.Linear(4, 8))),
                )
            else:
                self.module = nn.Sequential(
                    _maybe_wrap(nn.Linear(8, 4)),
                    _maybe_wrap(nn.Linear(4, 16)),
                    _maybe_wrap(nn.Linear(16, 4)),
                    _maybe_wrap(nn.Linear(4, 8)),
                )

    def get_input(self, device):
        torch.manual_seed(1 + self.rank)  # keep everything deterministic
        return (torch.rand(4, 8, device=device),)

    def forward(self, x):
        return self.module(x)

    def get_loss(self, input, output):
        loss = output.sum()
        return loss

    def run_backward(self, loss):
        loss.backward()


class DummyDDP(nn.Module):
    def __init__(self, module):
        super().__init__()
        self.module = module

    def forward(self, *args, **kwargs):
        return self.module(*args, **kwargs)


class MixtureOfExperts(NestedWrappedModule):
    def __init__(self, group, wrapper_config, checkpoint_act=False, delay_before_free_ms=0):
        super().__init__(group, wrapper_config)
        self.group = group
        self.delay_before_free_ms = delay_before_free_ms

        # "expert" params are different on each rank
        torch.manual_seed(42 + group.rank())
        d_expert = 23
        d_shared = 12
        d_input = 8
        expert = nn.Linear(d_expert, d_shared)

        self.num_expert_params = sum([p.numel() for p in expert.parameters()])
        for p in expert.parameters():
            p.expert = True

        # everything else is shared
        torch.manual_seed(0)

        shared = nn.Linear(d_shared, d_expert)

        if checkpoint_act:
            expert = checkpoint_wrapper(expert)
            shared = checkpoint_wrapper(shared)

        if wrapper_config is not None:
            # we create a process group of size 1 for the expert params
            expert_group = torch.distributed.new_group([group.rank()])  # world size 1 means no shard
            expert = FullyShardedDataParallel(expert, expert_group, **wrapper_config)

            shared = FullyShardedDataParallel(shared, group, **wrapper_config)

        self.module = nn.Sequential(nn.Linear(d_input, d_shared), shared, expert, nn.Linear(d_shared, d_input))

    def forward(self, x):
        if self.delay_before_free_ms > 0:
            expert = self.module[2]
            if isinstance(expert, FullyShardedDataParallel):
                orig_free_full_params = self.module[2]._free_full_params

                def _free_full_params_with_delay(*args):
                    torch.cuda._sleep(int(self.delay_before_free_ms * get_cycles_per_ms()))
                    return orig_free_full_params(*args)

                assert hasattr(expert, "_free_full_params")
                with mock.patch.object(expert, "_free_full_params", _free_full_params_with_delay):
                    return self.module(x)

        return self.module(x)

    def run_backward(self, loss):
        loss.backward()

        # manually reduce gradients if not wrapped in FullyShardedDataParallel
        if self.wrapper_config is None:
            with torch.no_grad():
                for p in self.parameters():
                    if hasattr(p, "expert"):
                        continue  # these params don't need grad reduction
                    p.grad.data.div_(self.world_size)
                    torch.distributed.all_reduce(p.grad.data, group=self.group)


class ModuleWithDelay(nn.Module):
    def __init__(self, module, delay_after_loss_ms=0, delay_before_reduction_ms=0):
        super().__init__()
        self.delay_after_loss_ms = delay_after_loss_ms
        self.delay_before_reduction_ms = delay_before_reduction_ms
        self.module = module

    def get_input(self, device):
        return self.module.get_input(device)

    def forward(self, x):
        return self.module(x)

    def get_loss(self, input, output):
        loss = self.module.get_loss(input, output)
        if self.delay_after_loss_ms > 0:
            torch.cuda._sleep(int(self.delay_after_loss_ms * get_cycles_per_ms()))
        return loss

    def run_backward(self, loss):
        orig_reduce_scatter = torch.distributed.reduce_scatter

        def _delayed_reduce_scatter(*args, **kwargs):
            if self.delay_before_reduction_ms > 0:
                torch.cuda._sleep(int(self.delay_before_reduction_ms * get_cycles_per_ms()))
            return orig_reduce_scatter(*args, **kwargs)

        with mock.patch("torch.distributed.reduce_scatter", _delayed_reduce_scatter):
            self.module.run_backward(loss)


class NestedWrappedModuleWithDelay(ModuleWithDelay):
    def __init__(self, group, wrapper_config, **kwargs):
        super().__init__(NestedWrappedModule(group, wrapper_config), **kwargs)


def spawn_and_init(fn, args=None, **spawn_kwargs):
    if args is None:
        args = ()

    run_fn = functools.partial(init_and_run, fn, args)
    spawn_for_all_world_sizes(run_fn, **spawn_kwargs)


def init_and_run(fn, args, rank, world_size, filename, filename_rpc):
    dist_init(rank, world_size, filename, filename_rpc)
    group = torch.distributed.new_group()
    fn(rank, group, *args)


if __name__ == "__main__":
    unittest.main()