[Llava][multimodal] enable Llava in torchchat

[Gasoonjia]

This PR aims to enable Llava in torchchat.

TODOs:

• [x] Create Model and ModelArgs as model definition entrances
• [x] Support model definition with multiple transformers
• [ ] Add transformer fusion types
• [ ] Enable Llava definition
• [ ] Add model config with checkpoint
• [ ] Update inference pipeline