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/

Here's a sample of how it's used:

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]

Noticed you were also making calls to HF models, you can simplify I/O to HF models using litellm too

example

completion(model="stabilityai/stablecode-completion-alpha-3b-4k", messages=messages, custom_llm_provider="huggingface")

More info here: https://docs.litellm.ai/docs/completion/supported#hugging-face-inference-api

[ishaan-jaff]

@virpo @mrshu can you take a look at this PR when possible😊

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