depois de muita pesquisa, tentativa e erro, compartilho os imports que consegui para realizar o exercício.
import os
from langchain_core.documents import Document
from langchain_google_genai import ChatGoogleGenerativeAI, GoogleGenerativeAIEmbeddings # LLM Gemini e embeddings
from langchain_community.vectorstores import FAISS
from langchain_community.retrievers import BM25Retriever
from langchain_classic.retrievers import EnsembleRetriever