[Bugfix] Solve ACLgraph automatically enabled problem

[lilinsiman]

What this PR does / why we need it?

Solve the problem that ACLgraph piecewise cannot be automatically enabled

Does this PR introduce any user-facing change?

no

How was this patch tested?

ut

• vLLM version: v0.10.2
• vLLM main: vllm-project/vllm@52d0cb8

[github-actions]

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[gemini-code-assist]

Code Review

This pull request introduces a fix to ensure ACLgraph piecewise is enabled correctly by downgrading CUDAGraphMode.FULL_AND_PIECEWISE to PIECEWISE for vLLM versions other than v0.10.2. While this addresses the immediate issue, the version check is brittle and may cause issues with future vLLM updates. I've added a comment with a suggestion to make the version check more robust for long-term maintainability.

[wangxiyuan]

fixed here #3182