[Misc] Add Gemma2 GGUF support

[Isotr0py]

FIX #12000 (link existing issues this PR will resolve)

[github-actions]

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[summersonnn]

@Isotr0py Hi. I couldn't wait until the merge and gave it a try. Got this:

File "/persistent/virtualenvs/llm/lib/python3.10/site-packages/torch/_tensor.py", line 983, in split
    return torch._VF.split_with_sizes(self, split_size, dim)
RuntimeError: split_with_sizes expects split_sizes to sum exactly to 8192 (input tensor's size at dimension -1), but got split_sizes=[8192, 4096, 4096]

during serving gemma-2-27b-it-Q4_K_M. any idea? I can post the full trace if you need.

vllm==0.6.6.post1 (+ your changes)
transformers==4.48.1
torch==2.5.1

[Isotr0py]

I see, seems that it's because of the incorrect head_dim due to gguf -> hf config conversion from transformers. The correct head_dim should be 128, while the value extracted from gguf is the default 256...