Update some parts of Functions, Tools and Agents with LangChain

[Beyondzjl]

1. 导入OpenAI的方式发生了变化：from langchain_openai import ChatOpenAI
2. ChatCompletion的调用方式发生变化：response = client.chat.completions.create(
   model="xx",
   messages = [
   {
   "role": "user",
   "content": "北京的天气怎么样?使用摄氏度作为单位。"
   }],
   functions=functions # 传入function参数
   )