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

[ishaan-jaff]

This PR adds support for 50+ models with a standard I/O interface using litellm: https://github.com/BerriAI/litellm/

ChatLiteLLM() is integrated into langchain and allows you to call all models using the ChatOpenAI 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]

@joshsny @Alec2435 can you please take a look at this PR when possible ? happy to add more docs/tests if this initial commit looks good 😊

[ghost]

bump @joshsny @Alec2435

[inferont]

@ishaan-jaff @krrishdholakia I'm open to a fork, #76

I'm not sure @joshsny and @Alec2435 want to maintain this repository.