src/transformers/tokenization_utils_fast.py/PreTrainedTokenizerFast/_add_tokens
class PreTrainedTokenizerFast:    def _add_tokens(self, new_tokens: List[Union[str, AddedToken]], special_tokens=False) -> int:
        if special_tokens:
            return self._tokenizer.add_special_tokens(new_tokens)

        return self._tokenizer.add_tokens(new_tokens)