langchain/chains/combine_documents/map_reduce.py/MapReduceDocumentsChain/output_keys
class MapReduceDocumentsChain:    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"]
        return _output_keys