ModernBert: reuse GemmaRotaryEmbedding via modular + Integration tests

[tomaarsen]

What does this PR do?

1. Add integration tests with the now-released checkpoint + 2 checkpoints under hf-internal-testing (thanks @xenova !)
2. Reuse GemmaRotaryEmbedding via modular functionality

Note that I based the expected values from the integration tests on the main implementation, not the RotaryEmbedding refactored implementation. In short: the test passes indicate that the refactor preserves the performance.

This is the refactor that I promised earlier offline.

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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• Did you write any new necessary tests?

Who can review?

@ArthurZucker

cc @warner-benjamin

• Tom Aarsen

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[warner-benjamin]

LGTM