src/transformers/tokenization_utils_base.py/SpecialTokensMixin/pad_token_id
class SpecialTokensMixin:    def pad_token_id(self) -> Optional[int]:
        """
        `Optional[int]`: Id of the padding token in the vocabulary. Returns `None` if the token has not been set.
        """
        if self._pad_token is None:
            return None
        return self.convert_tokens_to_ids(self.pad_token)