Reinstate self.scaling in Gemma3nTextAttention

[RyanMullins]

What does this PR do?

Previously, the Gemma3nTextAttention class removed the self.scaling property and passed 1.0 as a hard-coded value to the attention_interface() function.

This PR reinstates the self.scaling property, sets it to 1.0, and passes self.scaling to the attention_interface() function in Gemma3nTextAttention.forward(), which should make it more configurable for users interested in experimenting with the Gemma 3n arch, and improves the lineage of its modular inheritance relative to Gemma 3.

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@ArthurZucker @Cyrilvallez

[Rocketknight1]

Seems like a core question for text model philosophy so cc @ArthurZucker!

[github-actions]

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

run-slow: gemma3n

[Cyrilvallez]

LGTM, thanks!