[Chatllama] Add support to other open-source models

[diegofiori]

Description

LLaMA is a great model, however the weights license can be a huge barrier for users interested in the open-source LLM capabilities. The actual RLHF is designed around the LLaMA model, but it can be extended to other open-source models, like Flan-T5 or OPT.

TODO

• Extend support of ChatLLaMA to HuggingFace models
• Add support for the encoder-decoder architectures (AutoModelsForSeq2Seq)
• Add support for LLaMA from HF.
• Add support for Cerebras GPT.
• Test the support for
  • FlanT5
  • GPTJ
  • GPTNeoX
  • OPT
  • BLOOM
  • BLOOMZ
  • Galactica
• Implement unittest for the code