def pad_token(self):
        """ Padding token (string). Log an error if used while not having been set. """
        if self._pad_token is None:
            logger.error("Using pad_token, but it is not set yet.")
        return self._pad_token
 Padding token (string). Log an error if used while not having been set. 
xtune/src/transformers/tokenization_utils.py