# Copyright (c) 2017-present, Facebook, Inc.
# All rights reserved.
#
# This source code is licensed under the license found in the LICENSE file in
# the root directory of this source tree. An additional grant of patent rights
# can be found in the PATENTS file in the same directory.
#

from torch.nn.modules.loss import _Loss


class FairseqCriterion(_Loss):

    def __init__(self):
        super().__init__()

    def forward(self, model, sample):
        """Compute the loss for the given sample.

        Returns a tuple with three elements:
        1) the loss, as a Variable
        2) the sample size, which is used as the denominator for the gradient
        3) logging outputs to display while training
        """
        raise NotImplementedError

    @staticmethod
    def aggregate_logging_outputs(logging_outputs):
        """Aggregate logging outputs from data parallel training."""
        raise NotImplementedError

    @staticmethod
    def grad_denom(sample_sizes):
        """Compute the gradient denominator for a set of sample sizes."""
        return sum(sample_sizes)
