Fixes to alternating SWA layers in Gemma2

[turboderp]

What does this PR do?

• Reverses the order of global and sliding attention layers in Gemma2. This brings it in line with Google's implementation in which sliding attention is used on layers 0, 2, 4.., whereas currently the Transformers implementation uses sliding attn on layers 1, 3, 5...

• Changes HybridCache.update to read the sliding_window argument from cache_kwargs since it wasn't being parsed otherwise. The cache was created with alternating max seq lenghts of 4k and 8k, but all layers were being updated as if they were 8k, causing out-of-bounds errors and CUDA exceptions.

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Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR. @ArthurZucker

[LysandreJik]

Thanks for your PR @turboderp, we're taking a look with @ArthurZucker