langchain/chains/combine_documents/map_reduce.py/MapReduceDocumentsChain/acombine_docs
class MapReduceDocumentsChain:    async def acombine_docs(
        self, docs: List[Document], callbacks: Callbacks = None, **kwargs: Any
    ) -> Tuple[str, dict]:
        """Combine documents in a map reduce manner.

        Combine by mapping first chain over all documents, then reducing the results.
        This reducing can be done recursively if needed (if there are many documents).
        """
        results = await self.llm_chain.aapply(
            # FYI - this is parallelized and so it is fast.
            [{**{self.document_variable_name: d.page_content}, **kwargs} for d in docs],
            callbacks=callbacks,
        )
        return self._process_results(results, docs, callbacks=callbacks, **kwargs)