Add support for Palm, Claude-2, Cohere Llama2, CodeLlama (100+LLMs)

[ishaan-jaff]

This PR adds support for the above mentioned LLMs using LiteLLM https://github.com/BerriAI/litellm/
LiteLLM is a lightweight package to simplify LLM API calls - use any llm as a drop in replacement for gpt-3.5-turbo.

Example

from litellm import completion

## set ENV variables
os.environ["OPENAI_API_KEY"] = "openai key"
os.environ["COHERE_API_KEY"] = "cohere key"

messages = [{ "content": "Hello, how are you?","role": "user"}]

# openai call
response = completion(model="gpt-3.5-turbo", messages=messages)

# cohere call
response = completion(model="command-nightly", messages)

# anthropic call
response = completion(model="claude-instant-1", messages=messages)

Why are these changes needed?

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/FLAML/. See https://microsoft.github.io/FLAML/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[ishaan-jaff]

@sonichi @qingyun-wu can i get a review on this pr ?

happy to add docs/tests of this initial commit looks good to you

[qingyun-wu]

Hi @ishaan-jaff, Thanks for the contribution. This support looks appealing! But is it a good way to directly replace openai_completion? I am concerned about the potential unexpected consequences of this replacement. How about adding a LiteLLM option and using LiteLLM when the user explicitly configures so?