Pad labels on the length dimension to match logits length.

def pad_with_zeros(logits, labels):
  """Pad labels on the length dimension to match logits length."""
  with tf.name_scope("pad_with_zeros", values=[logits, labels]):
    logits, labels = pad_to_same_length(logits, labels)
    if len(labels.shape) == 3:  # 2-d labels.
      logits, labels = pad_to_same_length(logits, labels, axis=2)
    return logits, labels