src/transformers/models/marian/tokenization_marian.py/MarianTokenizer/_special_token_mask
class MarianTokenizer:    def _special_token_mask(self, seq):
        all_special_ids = set(self.all_special_ids)  # call it once instead of inside list comp
        all_special_ids.remove(self.unk_token_id)  # <unk> is only sometimes special
        return [1 if x in all_special_ids else 0 for x in seq]