Add support for Llama2, Palm, Cohere, Anthropic, Replicate, Azure 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")

Here's an example of how just using litellm.completion works

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"}]

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

# llama2 call
model_name = "replicate/llama-2-70b-chat:2c1608e18606fad2812020dc541930f2d0495ce32eee50074220b87300bc16e1"
response = completion(model_name, messages)

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

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

[ishaan-jaff]

@akshata29 can you please take a look at this PR ?😊

If this initial commit looks good happy to add liteLLM throughout + add docs/testing on the same

[akshata29]

@ishaan-jaff - Looks good to me, let me do a quick testing and appreciate if you can help liteLLM throughout + help add docs/testing.