src/transformers/tokenization_utils_base.py/SpecialTokensMixin/all_special_tokens
class SpecialTokensMixin:    def all_special_tokens(self) -> List[str]:
        """
        :obj:`List[str]`: All the special tokens (:obj:`'<unk>'`, :obj:`'<cls>'`, etc.) mapped to class attributes.

        Convert tokens of :obj:`tokenizers.AddedToken` type to string.
        """
        all_toks = [str(s) for s in self.all_special_tokens_extended]
        return all_toks