# Copyright (c) Alibaba, Inc. and its affiliates.
import torch
import torch.distributed as dist


class GatherLayer(torch.autograd.Function):
    '''Gather tensors from all process, supporting backward propagation.
    '''

    @staticmethod
    def forward(ctx, input):
        ctx.save_for_backward(input)
        output = [
            torch.zeros_like(input) for _ in range(dist.get_world_size())
        ]
        dist.all_gather(output, input)
        return tuple(output)

    @staticmethod
    def backward(ctx, *grads):
        input, = ctx.saved_tensors
        grad_out = torch.zeros_like(input)
        grad_out[:] = grads[dist.get_rank()]
        return grad_out