[CI Failure] Skip failing nvfp4 silu test

[mgoin]

Purpose

It seems the Blackwell Test is failing on main since #23929, so skipping if the symbol isn't found for now

Log: https://buildkite.com/vllm/ci/builds/28870/steps/canvas?sid=0198f6b1-3529-46d8-9896-06aac42648d6

FAILED tests/kernels/quantization/test_silu_nvfp4_quant_fusion.py::test_quantize_to_fp4[cuda:0-42-shape0-dtype0] - AttributeError: '_OpNamespace' '_C' object has no attribute 'silu_and_mul_nvfp4_quant'

Test Plan

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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• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request introduces a pragmatic fix for a CI failure in the nvfp4 SILU test. By checking for the existence of the silu_and_mul_nvfp4_quant operator before running the test, it ensures that the test suite doesn't fail in environments where this custom operator isn't compiled. My review includes a suggestion to update the test skip reason to accurately reflect this new condition, which will improve maintainability.

[DarkLight1337]

Maybe fixed by #23973, let's see https://buildkite.com/vllm/ci/builds/28921/steps/canvas?jid=0198f9d4-fc6b-4e72-89a5-08aa3fac4cd9

[njhill]

It appears this wasn't fixed by #23973 and it's still impacting CI. So we should probably merge this?

[yewentao256]

We should land this first, still PR failing like #23289