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

[ishaan-jaff]

I'm the maintainer of litellm https://github.com/BerriAI/litellm - a simple & light package to call OpenAI, Azure, Cohere, Anthropic API Endpoints

This PR does the following:

• Adds support for GPT-4 (via litellm)
• Simplify using anthropic - call it with the same Input as chatGPT and get output in the same format
• integrate litellm - which allows easily adding Azure, OpenAI, Palm, Anthropic, Cohere Models

Here's a sample of how litellm is used:

from litellm import completion

## 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"}]

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

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

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