参考：
https://github.com/langchain-ai/langchain
https://api.python.langchain.com/en/latest/community_api_reference.html
https://github.com/shibing624/ChatPilot/blob/384f18e4f10f87e10f104f9ff57f02c655588035/chatpilot/apps/rag_app.py
安装：
pip install langchain-community
pip install -U langchain
pip install pypdf
1、切分各种文档数据
from langchain_community.document_loaders imp
