langchain/chains/mapreduce.py/MapReduceChain/_call
class MapReduceChain:    def _call(
        self,
        inputs: Dict[str, str],
        run_manager: Optional[CallbackManagerForChainRun] = None,
    ) -> Dict[str, str]:
        _run_manager = run_manager or CallbackManagerForChainRun.get_noop_manager()
        # Split the larger text into smaller chunks.
        texts = self.text_splitter.split_text(inputs[self.input_key])
        docs = [Document(page_content=text) for text in texts]
        outputs = self.combine_documents_chain.run(
            input_documents=docs, callbacks=_run_manager.get_child()
        )
        return {self.output_key: outputs}