langchain/chains/combine_documents/stuff.py/StuffDocumentsChain/acombine_docs
class StuffDocumentsChain:    async def acombine_docs(
        self, docs: List[Document], callbacks: Callbacks = None, **kwargs: Any
    ) -> Tuple[str, dict]:
        """Stuff all documents into one prompt and pass to LLM."""
        inputs = self._get_inputs(docs, **kwargs)
        # Call predict on the LLM.
        return await self.llm_chain.apredict(callbacks=callbacks, **inputs), {}