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# Copyright (c) Facebook, Inc. and its affiliates.
#
# This source code is licensed under the MIT license found in the
# LICENSE file in the root directory of this source tree.
import torch
class LogSumExpMoE(torch.autograd.Function):
"""Standard LogSumExp forward pass, but use *posterior* for the backward.
See `"Mixture Models for Diverse Machine Translation: Tricks of the Trade"
(Shen et al., 2019) <https://arxiv.org/abs/1902.07816>`_.
"""
@staticmethod
def forward(ctx, logp, posterior, dim=-1):
ctx.save_for_backward(posterior)
ctx.dim = dim
return torch.logsumexp(logp, dim=dim)
@staticmethod
def backward(ctx, grad_output):
(posterior,) = ctx.saved_tensors
grad_logp = grad_output.unsqueeze(ctx.dim) * posterior
return grad_logp, None, None
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