Add support for Palm, Azure, Anthropic, Cohere, OpenAI, Replicate Models - using litellm

[ishaan-jaff]

This PR adds support for models from all the above mentioned providers using https://github.com/BerriAI/litellm - litellm allows you to interact with Palm, Azure, Anthropic, etc models using OpenAI chatCompletions Input/Output format.

It standardizes Inputs/outputs to LLM APIs making it easier to use newer models and debug easily

Here's a sample of how it's used:

from litellm import completion, acompletion

## set ENV variables
# ENV variables can be set in .env file, too. Example in .env.example
os.environ["OPENAI_API_KEY"] = "openai key"
os.environ["COHERE_API_KEY"] = "cohere key"

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

# google palm call
result = completion(model="chat-bison-001", messages=messages)

# google palm call
result = completion(model="text-bison-001", messages=messages)

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

# async openai
response = acompletion(model="gpt-3.5-turbo", messages=messages)

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

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

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[ishaan-jaff]

@michael-chi @giranntu can you please take a look at this ? If it looks good happy to add more docs/tests. Thought this would be useful because litellm simplifies using Non OpenAI LLM models😊

[shakeebshams]

Could you sign the cla: https://cla.developers.google.com/clas