Add vLLM-metax vllm metax cmake presets cudacxx

[ghangz]

Summary

• Adds a focused vllm metax cmake presets cudacxx improvement for MetaX-MACA/vLLM-metax.
• The change targets MetaX MACA development and validation workflows, with emphasis on earlier diagnostics, reproducible logs, or safer benchmark tooling.
• Existing default behavior is kept compatible; the new logic is scoped to explicit checks, helper tools, or validation metadata.

Validation

• Verified on Gitee.AI MetaX GPU resources: vLLM-metax_vLLM image batch, 10/10 PASS; PyTorch-MACA batch also covered vLLM-metax runtime tools.
• Branch validation command: python -m pytest tests/tools/test_generate_cmake_presets_cudacxx.py
• Pull request text is intentionally ASCII-only to avoid encoding issues on web forms and API clients.

Review notes

• Source branch: ghangz:mengz/vllm-metax-cmake-presets-cudacxx
• Target branch: MetaX-MACA/vLLM-metax:master
• Maintainers can modify this branch if follow-up adjustments are needed.

[gemini-code-assist]

Code Review

This pull request updates the CMake presets generation script to detect the CUDA compiler using the CUDACXX environment variable, prioritizing it over CUDA_HOME. A corresponding unit test was also added to verify this behavior. The review feedback suggests resolving CUDACXX using which if it is specified as a command name rather than an absolute path, ensuring that compilers in the system PATH are correctly detected.

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[gemini-code-assist]

If the CUDACXX environment variable is set to a compiler command name (e.g., nvcc or clang++) rather than an absolute path, os.path.exists(cudacxx) will return False. This causes the script to ignore the user's specified compiler and fall back to detecting nvcc elsewhere, which is incorrect. Resolving the compiler using which(cudacxx) when it is not a direct path ensures that command names in the system PATH are correctly detected and respected.

Suggested change

	cudacxx = os.environ.get("CUDACXX")
	if cudacxx and os.path.exists(cudacxx):
	nvcc_path = cudacxx
	print(f"Found CUDA compiler via CUDACXX: {nvcc_path}")
	cudacxx = os.environ.get("CUDACXX")
	if cudacxx:
	resolved_cudacxx = cudacxx if os.path.exists(cudacxx) else which(cudacxx)
	if resolved_cudacxx:
	nvcc_path = resolved_cudacxx
	print(f"Found CUDA compiler via CUDACXX: {nvcc_path}")