Compute loss numerator and denominator for one shard of output.

def generic_loss(top_out, targets, model_hparams, vocab_size, weights_fn):
  """Compute loss numerator and denominator for one shard of output."""
  del vocab_size  # unused arg
  logits = top_out
  logits = common_attention.maybe_upcast(logits, hparams=model_hparams)
  cutoff = getattr(model_hparams, "video_modality_loss_cutoff", 0.0)
  return common_layers.padded_cross_entropy(
      logits,
      targets,
      model_hparams.label_smoothing,
      cutoff=cutoff,
      weights_fn=weights_fn)