utils.py

from typing import Any, Optional, Tuple

import torch


class AllGatherGrad(torch.autograd.Function):
    # stolen from pytorch lightning
    @staticmethod
    def forward(
        ctx: Any,
        tensor: torch.Tensor,
        group: Optional["torch.distributed.ProcessGroup"] = None,
    ) -> torch.Tensor:
        ctx.group = group

        gathered_tensor = [
            torch.zeros_like(tensor) for _ in range(torch.distributed.get_world_size())
        ]

        torch.distributed.all_gather(gathered_tensor, tensor, group=group)
        gathered_tensor = torch.stack(gathered_tensor, dim=0)

        return gathered_tensor

    @staticmethod
    def backward(ctx: Any, *grad_output: torch.Tensor) -> Tuple[torch.Tensor, None]:
        grad_output = torch.cat(grad_output)

        torch.distributed.all_reduce(
            grad_output,
            op=torch.distributed.ReduceOp.SUM,
            async_op=False,
            group=ctx.group,
        )

        return grad_output[torch.distributed.get_rank()], None