fp16_torch.py

from typing import Callable, Optional, Tuple, Union

import torch
import torch.nn as nn
from torch import Tensor
from torch.optim import Optimizer

from colossalai.accelerator import get_accelerator
from colossalai.interface import ModelWrapper, OptimizerWrapper

from .mixed_precision_base import MixedPrecision

__all__ = ["FP16_Torch_MixedPrecision", "TorchAMPOptimizer", "TorchAMPModule"]


class TorchAMPOptimizer(OptimizerWrapper):
    """
    Optimizer wrapper for mixed precision training in FP16 using PyTorch AMP.

    Args:
        optim (Optimizer): Optimizer to wrap.
        init_scale (float): Initial scale factor. Default: 2**16.
        growth_factor (float): Factor by which the scale is multiplied during
            :meth:`torch.cuda.amp.GradScaler.step` if gradients were found to be finite
            this iteration. Default: 2.0.
        backoff_factor (float): Factor by which the scale is multiplied during
            :meth:`torch.cuda.amp.GradScaler.step` if gradients were found to be infinite
            this iteration. Default: 0.5.
        growth_interval (int): Number of iterations between :meth:`torch.cuda.amp.GradScaler.step`
            calls that may cause the scale to increase. Default: 2000.
    """

    def __init__(
        self,
        optim: Optimizer,
        init_scale: float = 2.0**16,
        growth_factor: float = 2.0,
        backoff_factor: float = 0.5,
        growth_interval: int = 2000,
    ) -> None:
        super().__init__(optim)
        self.scaler = torch.cuda.amp.GradScaler(
            init_scale=init_scale,
            growth_factor=growth_factor,
            backoff_factor=backoff_factor,
            growth_interval=growth_interval,
        )

    def backward(self, loss: Tensor, *args, **kwargs) -> None:
        scaled_loss = self.scale_loss(loss)
        scaled_loss.backward(*args, **kwargs)

    def step(self, *args, **kwargs) -> Optional[float]:
        out = self.scaler.step(self.optim, *args, **kwargs)
        self.scaler.update()
        return out

    def scale_loss(self, loss: Tensor) -> Tensor:
        return self.scaler.scale(loss)

    def unscale_grad(self) -> None:
        self.scaler.unscale_(self.optim)

    def clip_grad_by_value(self, clip_value: float, *args, **kwargs) -> None:
        self.unscale_grad()
        super().clip_grad_by_value(clip_value, *args, **kwargs)

    def clip_grad_by_norm(
        self,
        max_norm: Union[float, int],
        norm_type: Union[float, int] = 2.0,
        error_if_nonfinite: bool = False,
        *args,
        **kwargs,
    ) -> None:
        self.unscale_grad()
        super().clip_grad_by_norm(max_norm, norm_type, error_if_nonfinite, *args, **kwargs)


class TorchAMPModule(ModelWrapper):
    """
    Module wrapper for mixed precision training in FP16 using PyTorch AMP.

    Args:
        module (nn.Module): Module to wrap.
    """

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

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


class FP16TorchMixedPrecision(MixedPrecision):
    """
    Precision for mixed precision training in FP16 using PyTorch AMP.

    Args:
        init_scale (float): Initial scale factor. Default: 2**16.
        growth_factor (float): Factor by which the scale is multiplied during
            :meth:`torch.cuda.amp.GradScaler.step` if gradients were found to be finite
            this iteration. Default: 2.0.
        backoff_factor (float): Factor by which the scale is multiplied during
            :meth:`torch.cuda.amp.GradScaler.step` if gradients were found to be infinite
            this iteration. Default: 0.5.
        growth_interval (int): Number of iterations between :meth:`torch.cuda.amp.GradScaler.step`
            calls that may cause the scale to increase. Default: 2000.
    """

    def __init__(
        self,
        init_scale: float = 2.0**16,
        growth_factor: float = 2.0,
        backoff_factor: float = 0.5,
        growth_interval: int = 2000,
    ) -> None:
        super().__init__()
        self.torch_amp_kwargs = dict(
            init_scale=init_scale,
            growth_factor=growth_factor,
            backoff_factor=backoff_factor,
            growth_interval=growth_interval,
        )

    def configure(
        self,
        model: nn.Module,
        optimizer: Optional[Optimizer] = None,
        criterion: Optional[Callable] = None,
    ) -> Tuple[nn.Module, OptimizerWrapper, Callable]:
        model = TorchAMPModule(model)
        if optimizer is not None:
            optimizer = TorchAMPOptimizer(optimizer, **self.torch_amp_kwargs)
        if criterion is not None:
            criterion = TorchAMPModule(criterion)
        return model, optimizer, criterion