Experimental StableLM support

[kazssym]

What does this PR do?

This pull request tries to add support for the StableLM model. It is still incomplete.

Fixes # (issue)

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ ] Did you make sure to update the documentation with your changes?
• [ ] Did you write any new necessary tests?

[fxmarty]

@kazssym let me know if you'd like a review.

[xenova]

Can confirm that this PR works for transformers.js models 👍

@kazssym Just a reminder: you can add https://huggingface.co/hf-internal-testing/tiny-random-StableLmForCausalLM as a unit test.