langchain/indexes/vectorstore.py/VectorStoreIndexWrapper/query_with_sources
class VectorStoreIndexWrapper:    def query_with_sources(
        self, question: str, llm: Optional[BaseLanguageModel] = None, **kwargs: Any
    ) -> dict:
        """Query the vectorstore and get back sources."""
        llm = llm or OpenAI(temperature=0)
        chain = RetrievalQAWithSourcesChain.from_chain_type(
            llm, retriever=self.vectorstore.as_retriever(), **kwargs
        )
        return chain({chain.question_key: question})