vLLM AITER Patch Targets AMD Cliffs

This looks less like a model problem and more like a backend-selection problem: the faster ROCm path exists, but vLLM's defaults keep many users on slower attention kernels unless they opt in.

• –vLLM docs show `VLLM_ROCM_USE_AITER_UNIFIED_ATTENTION` defaults to false, even though the ROCm stack already has an AITER unified-attention backend.
• –The long-context collapse at roughly 64k tokens points to an attention-path bottleneck, not just a single bad GPU or a single model family.
• –The proposed RDNA4/gfx1201 patch path is straightforward in principle: relax arch gates, align MI350X assumptions, and keep FP16/BF16 KV cache.
• –Hybrid models like Qwen3 still stress unified-attention assumptions, especially around block sizing, so the backend is not a universal drop-in fix.
• –If the benchmark holds across more runs, AMD users should treat AITER support as a first-order performance requirement for long-context serving.