add: embedding model

[RyanMullins]

What does this PR do?

Adds support for embedding model use cases to Gemma 3.

• Adds use_bidirectional_attention config flag
• Adds _bidirectional_window_overlay() for local sliding attention
• Adds relevant or_mask functions
• Updates weights conversion script with entries for EmbeddingGemma and Gemma 3 270M

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• 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?

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@ArthurZucker @Cyrilvallez

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: gemma3

[Cyrilvallez]

LGTM! Thanks a lot! 🤗

[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.

[ArthurZucker]

Ty both! 🤗