Add support for Palm, Claude-2, Llama2, CodeLlama (100+LLMs)
Author: ishaan-jaff
Pull request: yes

# What does this PR do?
This PR adds support for the above mentioned LLMs using LiteLLM https://github.com/BerriAI/litellm/ 
Example
```python
# openai call
response = completion(model="gpt-3.5-turbo", messages=messages)
# cohere call
response = completion(model="command-nightly", messages=messages)
# anthropic
response = completion(model="claude-2", messages=messages)
```

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/ 

![liteLLM-Ab-testing](https://github.com/ConvLab/ConvLab-3/assets/29436595/4206cb62-4c83-4ea1-9be2-e5a7dd306048)

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Fixes # (issue)


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Models:

- text models: @ArthurZucker and @younesbelkada
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Library:

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- tokenizers: @ArthurZucker
- trainer: @muellerz and @pacman100

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Comments:
@stevhliu @LysandreJik can you please take a look at this PR when possible ? Happy to add more docs/tests if this initial commit looks good😊

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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