    def _compute_loss(
        self, outputs, targets, masks=None, label_smoothing=0.0, name="loss", factor=1.0
    ):
        """
        outputs: batch x len x d_model
        targets: batch x len
        masks:   batch x len

        policy_logprob: if there is some policy
            depends on the likelihood score as rewards.
        """

        def mean_ds(x: Tensor, dim=None) -> Tensor:
            return (
                x.float().mean().type_as(x)
                if dim is None
                else x.float().mean(dim).type_as(x)
            )

        if masks is not None:
            outputs, targets = outputs[masks], targets[masks]

        if masks is not None and not masks.any():
            nll_loss = torch.tensor(0)
            loss = nll_loss
        else:
            logits = F.log_softmax(outputs, dim=-1)
            if targets.dim() == 1:
                losses = F.nll_loss(logits, targets.to(logits.device), reduction="none")

            else:  # soft-labels
                losses = F.kl_div(logits, targets.to(logits.device), reduction="none")
                losses = losses.sum(-1)

            nll_loss = mean_ds(losses)
            if label_smoothing > 0:
                loss = (
                    nll_loss * (1 - label_smoothing) - mean_ds(logits) * label_smoothing
                )
            else:
                loss = nll_loss

        loss = loss * factor
        return {"name": name, "loss": loss, "nll_loss": nll_loss, "factor": factor}
outputs: batch x len x d_model
targets: batch x len
masks:   batch x len

policy_logprob: if there is some policy
depends on the likelihood score as rewards.
fairseq/criterions/nat_loss.py