Added Support for Multiple LLMs

[bshikin]

1. Created an abstract class LLM
2. Moved ChatGPTMixin to ChatGptLLM
3. Added support for various LLM configuration via following config strucdture:

   llm:
   class: ChatGptLLM
   config:
   model: "gpt-3.5-turbo"
   max_tokens: 4000

4. Multiple code changes to support the above