[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

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchchat/966

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As of commit 11113ff322b63327f0d19f6ab84bfb9ce29c9119 with merge base 9dc9effd674463b572125d01760cb78752e1912a (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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