langchain/chains/combine_documents/stuff.py/StuffDocumentsChain/_get_inputs
class StuffDocumentsChain:    def _get_inputs(self, docs: List[Document], **kwargs: Any) -> dict:
        # Format each document according to the prompt
        doc_strings = [format_document(doc, self.document_prompt) for doc in docs]
        # Join the documents together to put them in the prompt.
        inputs = {
            k: v
            for k, v in kwargs.items()
            if k in self.llm_chain.prompt.input_variables
        }
        inputs[self.document_variable_name] = self.document_separator.join(doc_strings)
        return inputs