[Hardware][AMD][Bugfix] Fix spec decode acceptance length test

[mawong-amd]

Purpose

This PR fixes the speculative decoding acceptance length tests, first implemented in #32030, to run on ROCm.

The current implementation fails because it erroneously calls current_platform.get_valid_backends which is not implemented on non-CUDA platforms. It guards this call behind hasattr(current_platform, "get_valid_backends"), but this does not work because the Platform class implements a __getattr__ method which means hasattr(current_platform, attribute) will trivially always return True for any attribute, even though the value of said attribute may be None.

Test Plan

pytest -sv tests/v1/spec_decode/test_acceptance_length.py on AMD ROCm.

Test Result

The above test passes on MI300X and MI325X.

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @mawong-amd.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[gemini-code-assist]

Code Review

This pull request introduces fixes to enable speculative decoding acceptance length tests on ROCm platforms. The key changes include correcting the attention backend detection logic in test_acceptance_length.py by replacing a faulty hasattr check and adding a specific path for ROCm. Additionally, a bug in eagle.py's dummy_run is fixed to correctly handle cases where CUDA graphs are disabled. The changes are logical, well-targeted, and improve compatibility with ROCm. I have reviewed the changes and they look good to me.

[mawong-amd]

Closing as a duplicate of #32787