Support BaiLing model

[cuauty]

We hope make our LLM named BaiLing become the one of the optional LLM on the chat.lmsys.org and join the chat on the website. We setup our own http end-point for LLM's reasoning and Bailing LLM has already been compatible with openai client and I have passed the test based on the FastChat document on my local environment.

I can setup PR to submit my code to fastchat/serve/api_provider.py. Is that all I need to do? Thank you.