langchain/chains/combine_documents/map_rerank.py/MapRerankDocumentsChain/output_keys
class MapRerankDocumentsChain:    def output_keys(self) -> List[str]:
        """Expect input key.

        :meta private:
        """
        _output_keys = super().output_keys
        if self.return_intermediate_steps:
            _output_keys = _output_keys + ["intermediate_steps"]
        if self.metadata_keys is not None:
            _output_keys += self.metadata_keys
        return _output_keys