Average log-perplexity with custom targets_mask.

def padded_neg_log_perplexity_with_masking(
    predictions,
    labels,
    features,
    weights_fn=None):
  """Average log-perplexity with custom targets_mask."""
  del weights_fn
  if "targets_mask" not in features:
    raise ValueError("masked_neg_log_perplexity requires targets_mask feature")

  # Features are 4 dimensional, so we need to reshape the targets_mask to match
  # the shape of the labels. A lot of models rely on these features being 4D,
  # so it's best to update the shape of the mask.
  extended_targets_mask_shape = common_layers.shape_list(
      features["targets_mask"])
  extended_targets_mask_shape.extend([1, 1])
  features["targets_mask"] = tf.reshape(features["targets_mask"],
                                        shape=extended_targets_mask_shape)

  mask_fn = lambda labels: features["targets_mask"]
  return padded_neg_log_perplexity(predictions, labels, mask_fn)