[Model][AMD] ROCm support for 256 head dims for Gemma

[jamestwhedbee]

Thanks to @jpvillam-amd's contributions on #3643, Gemma should have ROCm support

In my testing, I had to make these additional but trivial tweaks to actually use google/gemma-2b-it

This was tested on an MI100

FIX #3073 (link existing issues this PR will resolve)

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[WoosukKwon]

@jpvillam-amd Is head size 256 supported by the Triton kernel?

[shajrawi]

@WoosukKwon it is. We fully support this PR. Tested Triton with head sizes up to 256. In fact, this resolves #3073

[shajrawi]

Fixes #3073 - LGTM

[WoosukKwon]

Thanks for the fix!