Update app.py

app.py

@@ -1,6 +1,7 @@
 from langchain.chains import ConversationalRetrievalChain
 from langchain.chains.question_answering import load_qa_chain
 from langchain.memory import ConversationBufferMemory
+from langchain.memory import ConversationTokenBufferMemory
 from langchain.llms import HuggingFacePipeline
 from langchain import PromptTemplate
 from langchain.embeddings import HuggingFaceEmbeddings