fix block_size fix for GDN to xpu update_block_size_for_backend

[xuechendi]

Original, this branch suggested to fix triton_attn with block_size = 64 for XPU. Which will impact any triton_attention use case.
This PR suggested to update in xpu update_block_size_for_backend since the block_size=64 only needed by GDN (xpu version) and limitation might be removed in future.

VLLM_WORKER_MULTIPROC_METHOD=spawn python3 examples/basic/offline_inference/generate.py --model Qwen/Qwen3.5-9B  --enforce-eager  --attention-backend=TRITON_ATTN --max-model-len 4096 --temperature 0

[github-actions]

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