fix: move dsv4-fp4-mi355x MTP scripts into fixed_seq_len/

[cquil11]

The dsv4-fp4-mi355x-{sglang,vllm,atom}-mtp MTP benchmark scripts sat at benchmarks/single_node/ instead of benchmarks/single_node/fixed_seq_len/ with every other fixed-seq-len script.

runners/launch_mi355x-amds.sh resolves fixed-seq-len scripts to single_node/fixed_seq_len/<base>_<framework>_mtp.sh, so the launcher could not find them. No launcher/config change needed — the move aligns the files with the path the existing infra already expects.

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Note

Low Risk
Path-only relocation of benchmark shell scripts; no serving flags or launcher logic changes.

Overview
Relocates the three DSv4 FP4 MI355X MTP benchmark shells (sglang, vllm, atom) from benchmarks/single_node/ into benchmarks/single_node/fixed_seq_len/, alongside the rest of the fixed-seq-len recipes.

runners/launch_mi355x-amds.sh already resolves MTP jobs to single_node/fixed_seq_len/<base>_<framework>_mtp.sh via SCENARIO_SUBDIR and SPEC_SUFFIX, so the scripts were previously in the wrong directory and the launcher could not find them. No launcher or config updates—only the file paths change.

^{Reviewed by Cursor Bugbot for commit 485c675. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

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感谢你的贡献！对于 vLLM 与 SGLang，请确保你的 recipe 与官方 vLLM recipes 和/或 SGLang cookbook 保持一致

• https://docs.sglang.io/cookbook/intro
• https://recipes.vllm.ai/

如果不一致，请先创建一个 PR，之后我们才能将你的单节点 PR 合并到 master 分支。让我们确保文档保持一流水准，使整个 ML 社区都能从你的辛勤工作中受益！谢谢

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• https://github.com/sgl-project/sglang/tree/main/docs_new

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