langchain

fd5f549a - demonstrate use of RetrievalQAWithSourcesChain.from_chain (#12235)

demonstrate use of RetrievalQAWithSourcesChain.from_chain (#12235) **Description:** Documents further usage of RetrievalQAWithSourcesChain in an existing test. I'd not found much documented usage of RetrievalQAWithSourcesChain and how to get the sources out. This additional code will hopefully be useful to other potential users of this retriever. **Issue:** No raised issue **Dependencies:** No new dependencies needed to run the test (it already needs `open-ai`, `faiss-cpu` and `unstructured`). Note - `make lint` showed 8 linting errors in unrelated files --------- Co-authored-by: richarda23 <richard.c.adams@infinityworks.com>