src/transformers/tokenization_utils_base.py/SpecialTokensMixin/cls_token
class SpecialTokensMixin:    def cls_token(self) -> str:
        """
        :obj:`str`: Classification token, to extract a summary of an input sequence leveraging self-attention along the
        full depth of the model. Log an error if used while not having been set.
        """
        if self._cls_token is None and self.verbose:
            logger.error("Using cls_token, but it is not set yet.")
            return None
        return str(self._cls_token)