src/transformers/tokenization_utils_base.py/SpecialTokensMixin/bos_token_id
class SpecialTokensMixin:    def bos_token_id(self) -> Optional[int]:
        """
        :obj:`Optional[int]`: Id of the beginning of sentence token in the vocabulary. Returns :obj:`None` if the token
        has not been set.
        """
        if self._bos_token is None:
            return None
        return self.convert_tokens_to_ids(self.bos_token)