[AMD][DeepSeek V4] Set SGLANG_OPT_FLASHMLA_SPARSE_PREFILL to false on hip code path

[1am9trash]

co-authored: @bingxche, @michaelzhang-ai

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CI States

Latest PR Test (Base): ❌ Run #28777920839
Latest PR Test (Extra): ❌ Run #28777920522

[gemini-code-assist]

Code Review

This pull request modifies the DeepSeek-V4 hook for ROCm/HIP platforms by removing the check for whether the SGLANG_OPT_FLASHMLA_SPARSE_PREFILL environment variable is set. This change unconditionally disables the sparse prefill kernel on ROCm/HIP, preventing users from explicitly overriding this behavior. The reviewer recommends restoring the check to allow explicit overrides as documented in the warning message.

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[michaelzhang-ai]

Post-merge validation note (full DSV4-Flash disagg matrix on image ...-mi35x-20260706, which contains this PR):

Run: https://github.com/sgl-project/sglang/actions/runs/28817206537

This PR does what it intends — every prefill + decode log across all 8 flash configs shows Disabling SGLANG_OPT_FLASHMLA_SPARSE_PREFILL ... on ROCm/HIP, i.e. sparse prefill is confirmed off at runtime.

However, DeepSeek-V4-Flash disagg is still failing, and with sparse prefill confirmed disabled the cause is now clearly a separate MORI/RDMA transport issue, not the sparse path:

[application][error] RegisterRdmaMemoryRegion failed! size:7345537024, accessFlag:15, errno:22 (Invalid argument)
Fatal Python error: Aborted   (sglang/srt/utils/watchdog.py)
→ prefill worker aborts → "PD path not serving"

This is uniform across all 8 flash configs (fp8/fp4 × base/mtp/dp8ep8/dp8ep8-mtp), so it's systematic — a ~6.8 GB RDMA memory-region registration failing with EINVAL. Likely needs the MORI/disagg owners; cc @HaiShaw @1am9trash @bingxche @yctseng0211.

Net: #30237 correctly disables sparse prefill on HIP, but does not by itself green the Flash disagg nightly because of this RDMA registration failure.