Update app.py

app.py

@@ -8,6 +8,7 @@ from langchain.vectorstores import Chroma
 from langchain.docstore.document import Document
 from langchain.embeddings import HuggingFaceInstructEmbeddings
 from langchain.chains.conversation.memory import ConversationBufferMemory
+from langchain.chains import RetrievalQAWithSourcesChain
 
 from langchain.chains.conversation.memory import ConversationEntityMemory
 from langchain.chains.conversation.prompt import ENTITY_MEMORY_CONVERSATION_TEMPLATE