[`FlexAttention`] Update gemma2 (#34942)
* update tests
* now maybe this fixes the previous fialing tests!
* nit default
* Update src/transformers/models/gemma2/modular_gemma2.py
Co-authored-by: Anton Vlasjuk <73884904+vasqu@users.noreply.github.com>
* fix-copies
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Co-authored-by: Anton Vlasjuk <73884904+vasqu@users.noreply.github.com>
