def all_special_tokens(self):
        """
        list: All the special tokens ('<unk>', '<cls>'...) corresponding to
            special token arguments in `__init__` (arguments end with '_end').
        """
        all_toks = []
        set_attr = self.special_tokens_map
        for attr_value in set_attr.values():
            all_toks = all_toks + (list(attr_value) if isinstance(attr_value, (
                list, tuple)) else [attr_value])
        all_toks = list(set(all_toks))
        return all_toks

        list: All the special tokens ('<unk>', '<cls>'...) corresponding to
            special token arguments in `__init__` (arguments end with '_end').
        
ptstructure/vqa/pytorchnlp/transformers/tokenizer_utils.py