Fix Gemma3 embedding scaling
Author: gau-nernst
Pull request: yes

# What does this PR do?

- Correctly apply embedding scaling per [official Gemma 3 impl](https://github.com/google-deepmind/gemma/blob/ebfa2c2bddcab1d7cc38b1dca0fe6d98e6e7df71/gemma/modules.py#L93-L96) (cast scale to BF16). Gemma 2 has this done in #29402
- Use non-persistent buffer instead of plain Python float to make it more torch.compile-friendly (I have encountered cases where this is necessary, though it would be too verbose to elaborate it here)

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@ArthurZucker @RyanMullins 

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