gemma2: add sliding window mask

[Nexesenex]

Expand the usable context from 5k to 8k.

https://github.com/ggerganov/llama.cpp/pull/8227

Additional commits :

fix data_swa uninitialized better naming add co-author

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[Nexesenex]

The PR works as it is beyond the previous 5k context cap for Gemma V2 softcap. Slaren just wants to simplify the code.

[LostRuins]

I'll think i will wait to merge this when it's merged in llama.cpp

[Nexesenex]

And merged it is, with a further drop in perplexity ! :D