langchain/chains/combine_documents/stuff.py/StuffDocumentsChain/combine_docs
class StuffDocumentsChain:    def combine_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 self.llm_chain.predict(callbacks=callbacks, **inputs), {}