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

[ishaan-jaff]

This PR adds support for the above mentioned LLMs using LiteLLM https://github.com/BerriAI/litellm/
Example

fromlitellmimportcompletion## set ENV variablesos.environ["OPENAI_API_KEY"] ="openai key"os.environ["COHERE_API_KEY"] ="cohere key"messages= [{ "content": "Hello, how are you?","role": "user"}]
# openai callresponse=completion(model="gpt-3.5-turbo", messages=messages)
# cohere callresponse=completion("command-nightly", messages)
# anthropic callresponse=completion(model="claude-instant-1", messages=messages)

[ishaan-jaff]

@yashbonde@vgulerianb can I get a review on this PR ?

happy to add docs/testing if this initial commit looks good

Noticed you're allowing users to build LLM apps and I thought having more LLM will improve ChainFury