[Perf] Change default CUDAGraphMode from PIECEWISE to FULL_AND_PIECEWISE

[mgoin]

Purpose

This PR proposes to enable using full cudagraphs by default in vLLM V1. Support for cudagraphs beyond piecewise only was added over the past months (notably #20059) and while the startup time increase is measurable, we believe the performance gain for full cudagraphs is worth it, especially for low latency with small models or MoEs.

For instance, running Qwen/Qwen3-Coder-30B-A3B-Instruct-FP8 on 1xH100 with vLLM for 10 requests of 1024 in and 128 out results in the following throughputs:

• vllm serve (default): 4.07 req/s
• vllm serve --async-scheduling: 4.29 req/s
• vllm serve -O.cudagraph_mode=FULL: 5.83 req/s
• vllm serve --async-scheduling -O.cudagraph_mode=FULL: 6.03 req/s
• vllm serve -O.cudagraph_mode=FULL_AND_PIECEWISE: 5.98 req/s
• vllm serve --async-scheduling -O.cudagraph_mode=FULL_AND_PIECEWISE: 6.20 req/s

Benchmark command: vllm bench serve --model Qwen/Qwen3-Coder-30B-A3B-Instruct-FP8 --port 8000 --num-prompts 10

====

Startup time impact:

main: python -c "from vllm import LLM;LLM('Qwen/Qwen3-0.6B')"  34.05s user 7.32s system 190% cpu 21.684 total
pr:   python -c "from vllm import LLM;LLM('Qwen/Qwen3-0.6B')"  34.99s user 9.01s system 175% cpu 25.005 total

Test Plan

Test Result

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[gemini-code-assist]

Code Review

This pull request changes the default CUDA graph mode for the v1 engine from PIECEWISE to FULL_AND_PIECEWISE when using piecewise compilation. This is a performance optimization, as FULL_AND_PIECEWISE can leverage full CUDA graphs for decode steps, which is often more efficient. The change is implemented by updating the default-setting logic in VllmConfig.__post_init__. Correspondingly, docstrings in CompilationConfig have been updated to reflect that FULL_AND_PIECEWISE is now the default mode. The changes are logical, self-contained, and appear to be correct. I have not identified any critical or high-severity issues.

[mgoin]

Please wait on this, the time to capture the (decode, FULL) cudagraphs is very high. Like an order of magnitude more than the (mixed prefill-decode, PIECEWISE) or (mixed prefill-decode, FULL). We should get this down first.

[gau-nernst]

I was just reading up on this and tried out FULL_AND_PIECEWISE today. And it's already becoming the new default. Awesome and thank you 🙏

[LucasWilkinson]

Overall LGTM thanks! left one nit