[ROCm][CI] Fix test_cudagraph_mode failure in AMD CI

[micah-wil]

We are seeing failures in the tests/v1/cudagraph/test_cudagraph_mode.py test in AMD CI after #26980 was merged. It fails because it reaches the error "V0 attention backends have been removed. Set VLLM_USE_V1=1 to select a supported backend" since the test tries to use the FlashAttn backend. I updated the test to test ROCm attention backends if current_platform.is_rocm().

After this PR, we see:

pytest -v -s v1/cudagraph/test_cudagraph_mode.py:

=================================================== warnings summary ====================================================
<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: builtin type SwigPyPacked has no __module__ attribute

<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: builtin type SwigPyObject has no __module__ attribute

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
================================= 12 passed, 4 skipped, 2 warnings in 150.51s (0:02:30) =================================

[gemini-code-assist]

Code Review

This pull request resolves a CI failure on ROCm by implementing a fallback to the Triton attention backend when an unsupported backend is selected, instead of raising a RuntimeError. This is a sensible approach to make the system more robust. My review includes a suggestion to refine the warning message to be more specific about the reason for the fallback, which will improve clarity and aid in future debugging efforts.

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[tjtanaa]

Thank you for the fix.

[micah-wil]

Hey @tjtanaa, I have updated the test_cudagraph_mode test itself to test ROCm attention backends. I also reverted the change of defaulting to TritonAttn when trying to use an invalid attention backend. Could you please take another look? Thanks

cc @ProExpertProg