[Bugfix][Hardware][AMD] Skip MiniMax-M3 AITER sparse PA under spec decode

[seuthootDev]

Summary

• Disable the MiniMax-M3 AITER sparse PA prototype when speculative decoding is enabled. It silently corrupts output (repetition / mojibake) while the Triton sparse path does not ([Bug]: MiniMax-M3 AITER sparse PA prototype corrupts output under speculative decoding with FP8 KV (ROCm) #52860).
• Keep the AITER fast path for non-speculative serving. The gate is shared by impl selection and KV layout so Triton is not paired with AITER's separated K/V storage.
• The DSpark / VLLM_USE_BREAKABLE_CUDAGRAPH padding assert from the issue is out of scope and should stay a separate bug.

Duplicate-work check

• Issue [Bug]: MiniMax-M3 AITER sparse PA prototype corrupts output under speculative decoding with FP8 KV (ROCm) #52860 is open; no review comments proposing an overlapping fix.
• gh pr list --repo vllm-project/vllm --state open --search "52860 in:body": no open PRs.
• Nearby MiniMax/ROCm PRs ([Performance][ROCm] Integrate aiter indexer scoring and top-k kernels into MiniMax-M3 sparse attention path #52664, [ROCm][Perf] Fold scalar KV scales in MiniMax-M3 sparse attention #52060, [ROCm][Quant] Requantize serialized MXFP8 linears to FP8 PTPC #48427, [ROCm] Support MiniMax-M3 NVFP4 SwiGLU-OAI #48939) do not skip AITER sparse PA under spec decode.

Test plan

• python -m pytest tests/kernels/attention/test_minimax_m3_aiter_sparse_pa_selection.py -v --noconftest — 2 passed (CPU)
• ruff check and ruff format --check on the changed files
• tests/kernels/attention/test_minimax_m3.py AITER layout tests (need CUDA/ROCm)
• MI325X E2E: MiniMax-M3 FP8-dynamic TP8 + DSpark num_speculative_tokens=8, VLLM_ROCM_USE_AITER=1, VLLM_ROCM_SHUFFLE_KV_CACHE_LAYOUT=1. Greedy Count from 1 to 10 separated by commas: should emit 1,2,3,4,5,6,7,8,9,10 and log the Triton fallback warning.

No model-eval numbers from this environment: there is no ROCm/MI325X GPU here. The issue author's Triton-path numbers (44% acceptance, mean accepted length 4.55) are the expected E2E target after fallback.

AI assistance

AI assistance was used to implement the gate, tests, and this PR. I reviewed the changed lines. This is not a kernel rewrite; it is a fail-closed skip of a prototype that the issue already identified as unsafe under spec decode.

Fixes #52860

[claude]

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merged. Please rebase the PR, @seuthootDev.

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