Update app.py

app.py

@@ -11,6 +11,7 @@ from langchain.chains.question_answering import load_qa_chain
 from langchain_community.llms import huggingface_hub
 from langchain.document_loaders.pdf import PyMuPDFLoader 
 from transformers import AutoTokenizer, AutoModelForCausalLM
+from langchain_core.vectorstores import VectorStoreRetriever
 #from langchain.llms import huggingface_endpoint
 import os
 #import fitz