src/transformers/tokenization_utils_base.py/BatchEncoding/is_fast
class BatchEncoding:    def is_fast(self) -> bool:
        """
        :obj:`bool`: Indicate whether this :class:`~transformers.BatchEncoding` was generated from the result of a
        :class:`~transformers.PreTrainedTokenizerFast` or not.
        """
        return self._encodings is not None