Add support for 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/

In addition LiteLLM client allows you to:

• A/B test LLMs in production
• Dynamically control each LLMs prompt, temperature, top_k etc in our UI (no need to re-deploy code)
• Logging to view input/outputs for each LLM

Here's a link to a live demo of litellm client: https://admin.litellm.ai/

[ishaan-jaff]

@irgolic can I get a review on this PR ?

Happy to add docs/tests if this initial commit looks good😊