[AMD][V4.1][*/N] Switch the fp8 dense GEMMs on gfx950 to aiter's MXFP8 GEMM

[RolaoDenthu]

Motivation

On gfx950, the DeepSeek-V4.1 fp8 dense linears (32x32-block weights with ue8m0 scales) run on sglang's native MXFP8 route.

ROCm/aiter#5750 adds a native MXFP8 GEMM to aiter (e4m3 with e8m0 scales per 32 along K, 32x32-block weight scales), reached through aiter.gemm_a8w8_blockscale. This PR adds it as an opt-in route: with --fp8-gemm-backend aiter, the V4.1 fp8 dense linears on gfx950 run on aiter's kernel.

Modifications

Add aiter's MXFP8 GEMM as a gfx950 backend for the DeepSeek-V4.1 fp8 dense linears, plus tests.

• Opt-in with --fp8-gemm-backend aiter
• Test: TestMxfp8AiterRouteGfx95 in test_mxfp8_amd_gfx95.py.

Accuracy Tests

gsm8k 1319: 0.909

Speed Tests and Profiling

kernel level test

With DSpark (block size 5), decode M is 6 x concurrency.

M	concurrency	native	aiter	aiter / native
6	1	35.4	37.2	1.05x
12	2	39.2	39.8	1.02x
24	4	58.1	37.6	0.65x
48	8	106.2	45.3	0.43x
96	16	109.3	52.8	0.48x
192	32	116.2	64.1	0.55x
512	prefill	148.0	105.7	0.71x

e2e test

8k1k

Concurrency	Metric	Baseline (native)	aiter	Change
1	TTFT (ms)	400.29	396.40	−1.0%
1	TPOT (ms)	1.66	1.66	0
1	ITL (ms)	1.66	1.66	0
1	Output throughput (tok/s)	411.46	430.21	+4.6%
4	TTFT (ms)	425.99	420.38	−1.3%
4	TPOT (ms)	3.76	3.59	−4.5%
4	ITL (ms)	2.68	2.51	−6.3%
4	Output throughput (tok/s)	839.72	875.57	+4.3%
16	TTFT (ms)	1096.42	943.27	−14.0%
16	TPOT (ms)	9.39	9.00	−4.2%
16	ITL (ms)	4.67	4.31	−7.7%
16	Output throughput (tok/s)	1349.40	1414.23	+4.8%

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
• Follow the SGLang code style guidance.

Review and Merge Process

1. Ping Merge Oncalls to start the process. See the PR Merge Process.
2. Get approvals from CODEOWNERS and other reviewers.
3. Trigger CI tests with comments or contact authorized users to do so.
   • Common commands include /tag-and-rerun-ci, /tag-run-ci-label, /rerun-failed-ci
4. After green CI and required approvals, ask Merge Oncalls or people with Write permission to merge the PR.

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CI States

Latest PR Test (Base): ❌ Run #36810199110
Latest PR Test (Extra): ❌ Run #36810198773
Latest PR Test (AMD ROCm 10): ❌ Run #36810198918