op.py

# Copyright (c) 2022-2024, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
#
# See LICENSE for license information.

"""Base classes for fusible operations."""

from __future__ import annotations
import abc
from collections.abc import Iterable
import dataclasses
from typing import Any, Optional

import torch

import transformer_engine_torch as tex
from transformer_engine.pytorch.fp8 import (
    FP8GlobalStateManager,
    get_default_fp8_recipe,
)
from ._common import canonicalize_device, is_float8_tensor


@dataclasses.dataclass
class OperationContext:
    """State needed to apply an operation

    Saves state from forward pass for use in backward pass.

    """

    # Tensors that have been saved from forward function
    # Note: Available in the backward function, matching tensors from
    # to_save.
    saved_tensors: Optional[tuple[Optional[torch.Tensor], ...]] = None
    # Tensors to save for backward function
    # Note: Expected to be set in the forward function, either
    # directly or with save_for_backward.
    to_save: Optional[tuple[Optional[torch.Tensor], ...]] = None

    # Corresponding range in pipeline's list of saved tensors
    _saved_tensors_range: Optional[tuple[int, int]] = None

    # Whether backward pass is required
    _requires_grad: bool = False

    def save_for_backward(self, *tensors: Optional[torch.Tensor]) -> None:
        """Register tensors to be saved for the backward function

        Expected to be called in the forward function.

        """
        self.to_save = tensors


class FusibleOperation(torch.nn.Module, metaclass=abc.ABCMeta):
    """Tensor operation supported by the operation fuser"""

    @property
    @abc.abstractmethod
    def is_fused_op(self) -> bool:
        """Whether this op is the fusion of one or more basic ops"""

    def pre_forward(self) -> None:
        """Preprocessing before forward pass"""

    def fuser_forward(
        self,
        basic_op_ctxs: list[OperationContext],
        input_: torch.Tensor,
        basic_op_prev_ops: list[Optional[BasicOperation]],
        basic_op_next_ops: list[Optional[BasicOperation]],
        basic_op_kwargs: list[dict[str, Any]],
    ) -> torch.Tensor:
        """Forward pass

        This op is either a basic op or the fusion of basic ops, so
        several of this function's arguments are lists of arguments to
        forward functions of corresponding basic ops.

        Called by `OperationFuser`.

        Parameters
        ----------
        basic_op_ctxs: list of OperationContext
            Contexts for corresponding basic operations
        input_: torch.Tensor
            Input tensor
        basic_op_prev_ops: list of BasicOperation
            Basic operations that preceed each of the corresponding
            basic operations (or `None` if corresponding basic op is
            first)
        basic_op_next_ops: list of BasicOperation
            Basic operations that follow each of the corresponding
            basic operations (or `None` if corresponding basic op is
            last)
        basic_op_kwargs: list of dict
            Keyword arguments to forward functions of corresponding
            basic operations

        Returns
        -------
        torch.Tensor: Output tensor.

        """
        raise NotImplementedError(
            f"Forward pass is not implemented for operation ({self.__class__.__name__})"
        )

    def fuser_backward(
        self,
        basic_op_ctxs: list[OperationContext],
        grad_output: torch.Tensor,
    ) -> tuple[torch.Tensor, Iterable[Iterable[Optional[torch.Tensor]]]]:
        """Backward pass

        This op is either a basic op or the fusion of basic ops, so
        several of this function's arguments are lists of arguments to
        backward functions of corresponding basic ops.

        Called by `OperationFuser`.

        Parameters
        ----------
        basic_op_ctxs: list of OperationContext
            Contexts for corresponding basic operations.
        grad_output: torch.Tensor
            Loss gradient w.r.t. operation output.
        basic_op_prev_ops: list of BasicOperation
            Basic operations that preceed each of the corresponding
            basic operations (or `None` if corresponding basic op is
            first)
        basic_op_next_ops: list of BasicOperation
            Basic operations that follow each of the corresponding
            basic operations (or `None` if corresponding basic op is
            last)

        Returns
        -------
        torch.Tensor:
            Loss gradient w.r.t. operation input
        Iterable of iterable of torch.Tensor:
            Loss gradients w.r.t. parameters for corresponding basic
            operations

        """
        raise NotImplementedError(
            f"Backward pass is not implemented for operation ({self.__class__.__name__})"
        )


class BasicOperation(FusibleOperation, metaclass=abc.ABCMeta):
    """Single tensor operation supported by the operation fuser

    This class holds parameters and state, even if the actual forward
    and backward passes are performed by a fused operation.

    """

    def __init__(self) -> None:
        super().__init__()

        # FP8 metadata objects
        self._fp8_metas: Optional[dict[str, dict[str, Any]]] = None

    @property
    def is_fused_op(self) -> bool:
        return False

    # pylint: disable=no-self-use
    def num_fp8_scales(
        self,
        mode: str,  # pylint: disable=unused-argument
    ) -> int:
        """Number of FP8 scaling factors

        Parameters
        ----------
        mode: {"input", "param", "grad_output"}
            Type of FP8 scaling factor

        """
        return 0

    def _make_fp8_metas(self) -> dict[str, Optional[dict[str, Any]]]:
        """Construct FP8 metadata"""

        # Shared objects for FP8 metadata
        dtype = torch.float32
        device = canonicalize_device(None)
        recipe = get_default_fp8_recipe()

        def _make_meta(
            num_scales: int,
            is_forward: bool,
        ) -> Optional[dict[str, Any]]:
            """Construct FP8 metadata for one tensor type"""
            if num_scales == 0:
                return None
            key = FP8GlobalStateManager.get_meta_tensor_key(forward=is_forward)
            meta = tex.FP8TensorMeta()
            meta.scale = torch.ones(num_scales, dtype=dtype, device=device)
            meta.scale_inv = torch.ones(num_scales, dtype=dtype, device=device)
            meta.amax_history = torch.zeros(
                (recipe.amax_history_len, num_scales),
                dtype=dtype,
                device=device,
            )
            return {
                key: meta,
                "recipe": recipe,
                "fp8_group": None,
            }

        # Construct FP8 metadata for all tensor types
        return dict(
            input=_make_meta(self.num_fp8_scales("input"), True),
            param=_make_meta(self.num_fp8_scales("param"), True),
            grad_output=_make_meta(self.num_fp8_scales("grad_output"), False),
        )

    @classmethod
    def _maybe_update_fp8_meta(cls, fp8_meta: Optional[dict[str, Any]]) -> None:
        if fp8_meta is None:
            return

        # Update FP8 recipe and communication group
        recipe = FP8GlobalStateManager.get_fp8_recipe()
        fp8_meta["recipe"] = recipe
        fp8_meta["fp8_group"] = FP8GlobalStateManager.get_fp8_group()

        # Adjust amax history length if needed
        amax_history_len = recipe.amax_history_len
        for is_forward in (True, False):
            key = FP8GlobalStateManager.get_meta_tensor_key(forward=is_forward)
            if key not in fp8_meta:
                continue
            meta = fp8_meta[key]
            curr_len = meta.amax_history.size(0)
            if curr_len == amax_history_len:
                continue
            with torch.no_grad():
                if curr_len > amax_history_len:
                    meta.amax_history = meta.amax_history[:amax_history_len].clone()
                else:
                    meta.amax_history = torch.nn.functional.pad(
                        meta.amax_history,
                        pad=(0, 0, 0, amax_history_len - curr_len),
                    )

    def get_fp8_meta(self, mode: str) -> Optional[dict[str, Any]]:
        """FP8 metadata

        Parameters
        ----------
        mode: {"input", "param", "grad_output"}
            Type of FP8 scaling factor

        """
        if self._fp8_metas is None:
            self._fp8_metas = self._make_fp8_metas()
        return self._fp8_metas[mode]

    def pre_forward(self) -> None:
        """Preprocessing before forward pass"""

        # Initialize FP8 metadata if needed
        fp8_enabled = FP8GlobalStateManager.is_fp8_enabled()
        if fp8_enabled:

            # Construct FP8 metadata if needed
            if self._fp8_metas is None:
                self._fp8_metas = self._make_fp8_metas()

            # Make sure FP8 metadata matches FP8 autocast context
            for fp8_meta in self._fp8_metas.values():
                self._maybe_update_fp8_meta(fp8_meta)

            # Register FP8 metadata for amax and scale update
            if not FP8GlobalStateManager.fp8_graph_capturing():
                if self.num_fp8_scales("input"):
                    FP8GlobalStateManager.add_fp8_tensors_to_global_buffer(
                        self.get_fp8_meta("input"),
                    )
                if self.num_fp8_scales("param"):
                    fp8_params = list(filter(is_float8_tensor, self.parameters()))
                    FP8GlobalStateManager.add_fp8_tensors_to_global_buffer(
                        self.get_fp8_meta("param"),
                        fp8_weights=(fp8_params if fp8_params else None),
                    )
                if self.num_fp8_scales("grad_output"):
                    FP8GlobalStateManager.add_fp8_tensors_to_global_buffer(
                        self.get_fp8_meta("grad_output"),
                    )

    @abc.abstractmethod
    def op_forward(
        self,
        ctx: OperationContext,
        input_: torch.Tensor,
        prev_op: Optional[BasicOperation] = None,
        next_op: Optional[BasicOperation] = None,
        **kwargs: Any,
    ) -> torch.Tensor:
        """Forward pass

        Parameters
        ----------
        ctx: OperationContext
            Context to coordinate between forward and backward passes
        input_: torch.Tensor
            Input tensor

        Returns
        -------
        torch.Tensor:
            Output tensor

        """

    @abc.abstractmethod
    def op_backward(
        self,
        ctx: OperationContext,
        grad_output: torch.Tensor,
    ) -> tuple[torch.Tensor, Iterable[Optional[torch.Tensor]]]:
        """Backward pass

        Parameters
        ----------
        ctx: OperationContext
            Context to coordinate between forward and backward passes
        grad_output: torch.Tensor
            Loss gradient w.r.t. operation output

        Returns
        -------
        torch.Tensor
            Loss gradient w.r.t. operation input
        Iterable of torch.Tensor:
            Loss gradients w.r.t. parameters

        """

    def fuser_forward(
        self,
        basic_op_ctxs: list[OperationContext],
        input_: torch.Tensor,
        basic_op_prev_ops: list[Optional[BasicOperation]],
        basic_op_next_ops: list[Optional[BasicOperation]],
        basic_op_kwargs: list[dict[str, Any]],
    ) -> torch.Tensor:
        return self.op_forward(
            basic_op_ctxs[0],
            input_,
            basic_op_prev_ops[0],
            basic_op_next_ops[0],
            **basic_op_kwargs[0],
        )

    def fuser_backward(
        self,
        basic_op_ctxs: list[OperationContext],
        grad_output: torch.Tensor,
    ) -> tuple[torch.Tensor, Iterable[Iterable[Optional[torch.Tensor]]]]:
        grad_input, grad_params = self.op_backward(basic_op_ctxs[0], grad_output)
        return grad_input, [grad_params]

    def forward(
        self,
        input: torch.Tensor,  # pylint: disable=redefined-builtin
        **kwargs: Any,
    ) -> torch.Tensor:
        """Apply operation"""
        from .fuser import OperationFuser

        return OperationFuser([self], fuse_ops=False)(input, [kwargs])


class FusedOperation(FusibleOperation):
    """Compound tensor operation supported by the operation fuser

    If the forward or backward passes are defined, they must be
    functionally equivalent to the forward/backward passes of the
    corresponding basic ops. This class should hold no parameters or
    other state, but should access them from the basic ops.

    Parameters
    ----------
    basic_ops: iterable of FusibleOperation
        Basic ops that are interchangeable with this op

    """

    def __init__(
        self,
        basic_ops: Iterable[FusibleOperation],
    ) -> None:
        super().__init__()

        # Basic operations that comprise this fused operation
        self.basic_ops: torch.nn.ModuleList = torch.nn.ModuleList(basic_ops)
        if len(self.basic_ops) == 0:
            raise ValueError(
                "Attempted to construct a fused operation "
                "without specifying its corresponding basic operations"
            )

    @property
    def is_fused_op(self) -> bool:
        return True

    def pre_forward(self) -> None:
        """Preprocessing before forward pass"""
        for op in self.basic_ops:
            op.pre_forward()

    def forward(
        self,
        input: torch.Tensor,  # pylint: disable=redefined-builtin
        basic_op_kwargs: Optional[list[dict[str, Any]]] = None,
    ) -> torch.Tensor:
        """Apply operation"""
        if basic_op_kwargs is None:
            basic_op_kwargs = [{} for _ in range(len(self.basic_ops))]
        from .fuser import OperationFuser

        return OperationFuser([self], fuse_ops=False)(input, basic_op_kwargs)