feat(vllm): add Gemma 4 models, image, and ROCm serving recipes

[coketaste]

• Register pyt_vllm_gemma-4-26b-a4b-it and pyt_vllm_gemma-4-31b-it in models.json (gemma4 Docker stack).
• Add docker/pyt_vllm_gemma4.ubuntu.amd.Dockerfile from vllm/vllm-openai-rocm:gemma4 with transformers 5.5.0.
• Extend scripts/vllm/configs/default.yaml with Gemma 4 serving blocks (TRITON_ATTN, gfx942 float16; 26B MoE disables AITER fused MoE).
• Quote JSON-like extra_args in run_vllm.py (shlex) for --limit-mm-per-prompt with existing --flag YAML keys.
• Document Gemma 4 in benchmark/vllm/README.md.

[Copilot]

Pull request overview

Adds Gemma 4 (26B-A4B-it and 31B-it) vLLM serving support to the MAD benchmarking stack, including new model registrations, ROCm/Gemma4 Docker build plumbing, and documented serving recipes.

Changes:

• Registered two Gemma 4 vLLM models in models.json and documented them in benchmark/vllm/README.md.
• Added a Gemma4-specific AMD Ubuntu Dockerfile based on vllm/vllm-openai-rocm:gemma4 and extended scripts/vllm/configs/default.yaml with Gemma 4 serving recipes/overrides.
• Updated scripts/vllm/run_vllm.py to shell-quote JSON-like/whitespace-containing extra_args values (notably --limit-mm-per-prompt).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
scripts/vllm/run_vllm.py	Adjusts extra_args formatting/quoting when composing the vLLM command line.
scripts/vllm/configs/default.yaml	Adds Gemma 4 serving benchmark blocks and gfx942 dtype overrides.
models.json	Registers Gemma 4 vLLM models and their MAD metadata/output CSV names.
docker/pyt_vllm_gemma4.ubuntu.amd.Dockerfile	Introduces a Gemma4-tagged base image Dockerfile and pins transformers.
benchmark/vllm/README.md	Documents Gemma 4 image tag usage, gating/token requirements, and recipe details.

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