[Bugfix][CI] Move resolving cudagraph_mode before initializing attn_metadata_builder

[fhl2000]

Purpose

Previously, resolving cudagraph_mode happened after attn_metadata_builder initialisation, which means attn_metadata_builder is not aware of the potential changed cudagraph_mode. (could lead to querying invalid cudagraph_capturing_sizes list if the resolved mode is NONE).

This should also fix the CI for #26016

Test Plan

To see if the CI is fixed in #26016 after merging this.

Test Result

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
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[gemini-code-assist]

Code Review

This pull request correctly addresses a bug where cudagraph_mode was resolved after the initialization of attn_metadata_builder, which could lead to incorrect behavior. The fix refactors the initialization logic in GPUModelRunner to ensure that cudagraph_mode is determined based on attention backend capabilities before the metadata builders are instantiated. This is achieved by first collecting all attention backend classes, determining the minimum supported CUDA graph level, and only then initializing the attention groups. The changes are logical, well-implemented, and effectively resolve the issue. The associated refactoring improves the code's clarity and correctness. The changes look solid.

[mergify]

Documentation preview: https://vllm--27427.org.readthedocs.build/en/27427/