Add support for Llama2, Palm, Cohere, Anthropic, Replicate Models - using litellm

[ishaan-jaff]

This PR adds support for models from all the above mentioned providers using https://github.com/BerriAI/litellm/

ChatLiteLLM() is integrated into langchain and allows you to call all models using the OpenAI I/O interface
https://python.langchain.com/docs/integrations/chat/litellm

Here's an example of how to use ChatLiteLLM()

ChatLiteLLM(model="gpt-3.5-turbo")
ChatLiteLLM(model="claude-2", temperature=0.3)
ChatLiteLLM(model="command-nightly")
ChatLiteLLM(model="replicate/llama-2-70b-chat:2c1608e18606fad2812020dc541930f2d0495ce32eee50074220b87300bc16e1")

[ishaan-jaff]

@kaarthik108 can you please take a look at this PR when possible ?

Happy to add more docs/tests if this initial commit ooks good

[kaarthik108]

@ishaan-jaff Thank you, this looks great
can you also update the requirements.txt, looks like langchain needs to be upgraded and also additional dependencies ?
Would also be good if it works on llama 13b replicate model ?