Tensor with zeros in padding positions and ones elsewhere.

def nonpadding(self):
    """Tensor with zeros in padding positions and ones elsewhere."""
    if self.sequence_id is None:
      return None
    if self.sequence_id == 1:
      return 1
    else:
      return mtf.cast(
          mtf.not_equal(self.sequence_id, 0), self.activation_dtype)