chore: Update langchain_community.retrievers imports

app.py

@@ -5,7 +5,8 @@ import gradio as gr
 from dotenv import load_dotenv
 from langchain.callbacks.base import BaseCallbackHandler
 from langchain.embeddings import CacheBackedEmbeddings
-from langchain_community.retrievers import BM25Retriever, EnsembleRetriever
+from langchain_community.retrievers import BM25Retriever
+from langchain.retrievers import EnsembleRetriever
 from langchain.storage import LocalFileStore
 from langchain_anthropic import ChatAnthropic
 from langchain_community.chat_models import ChatOllama