    def forward(self, emissions, targets, masks, beam=None):
        """
        Compute the conditional log-likelihood of a sequence of target tokens given emission scores

        Args:
            emissions (`~torch.Tensor`): Emission score are usually the unnormalized decoder output
                ``(batch_size, seq_len, vocab_size)``. We assume batch-first
            targets (`~torch.LongTensor`): Sequence of target token indices
                ``(batch_size, seq_len)
            masks (`~torch.ByteTensor`): Mask tensor with the same size as targets

        Returns:
            `~torch.Tensor`: approximated log-likelihood
        """
        numerator = self._compute_score(emissions, targets, masks)
        denominator = self._compute_normalizer(emissions, targets, masks, beam)
        return numerator - denominator
Compute the conditional log-likelihood of a sequence of target tokens given emission scores

Args:
emissions (`~torch.Tensor`): Emission score are usually the unnormalized decoder output
``(batch_size, seq_len, vocab_size)``. We assume batch-first
targets (`~torch.LongTensor`): Sequence of target token indices
``(batch_size, seq_len)
masks (`~torch.ByteTensor`): Mask tensor with the same size as targets

Returns:
`~torch.Tensor`: approximated log-likelihood
fairseq/modules/dynamic_crf_layer.py