def mask_token(self):
        """ Mask token (string). E.g. when training a model with masked-language modeling. Log an error if used while not having been set. """
        if self._mask_token is None:
            logger.error("Using mask_token, but it is not set yet.")
        return self._mask_token
 Mask token (string). E.g. when training a model with masked-language modeling. Log an error if used while not having been set. 
xtune/src/transformers/tokenization_utils.py