    def get_empty_token_list(self) -> IndexedTokenList:
        """
        Returns an `already indexed` version of an empty token list.  This is typically just an
        empty list for whatever keys are used in the indexer.
        """
        raise NotImplementedError
Returns an `already indexed` version of an empty token list.  This is typically just an
empty list for whatever keys are used in the indexer.
allennlp/data/token_indexers/token_indexer.py