Added `.as_retriever()` to `from_llm()` calls (#2051)

docs/modules/chains/index_examples/chat_vector_db.ipynb

@@ -121,7 +121,7 @@
    },
    "outputs": [],
    "source": [
-    "qa = ConversationalRetrievalChain.from_llm(OpenAI(temperature=0), vectorstore)"
+    "qa = ConversationalRetrievalChain.from_llm(OpenAI(temperature=0), vectorstore.as_retriever())"
    ]
   },
   {
@@ -230,7 +230,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "qa = ConversationalRetrievalChain.from_llm(OpenAI(temperature=0), vectorstore, return_source_documents=True)"
+    "qa = ConversationalRetrievalChain.from_llm(OpenAI(temperature=0), vectorstore.as_retriever(), return_source_documents=True)"
    ]
   },
   {
@@ -292,7 +292,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "qa = ConversationalRetrievalChain.from_llm(OpenAI(temperature=0), vectorstore, return_source_documents=True)\n",
+    "qa = ConversationalRetrievalChain.from_llm(OpenAI(temperature=0), vectorstore.as_retriever(), return_source_documents=True)\n",
     "chat_history = []\n",
     "query = \"What did the president say about Ketanji Brown Jackson\"\n",
     "result = qa({\"question\": query, \"chat_history\": chat_history, \"vectordbkwargs\": vectordbkwargs})"