[ROCm] Attention selector reordering

[gshtras]

With the unit tests now able to handle this change following
#36025 #35334 and others
Changing the priorities of ROCm attention backends to

1. ROCM_ATTN - when applicable it is the most performant backend today
2. AITER_MHA - when explicitly selected
3. AITER_UNIFIED - a variation of TRITON_ATTN, specifically tuned for ROCm. Will fall back here when the model requires sinks (GPT-OSS)
4. TRITON_ATTN - the most versatile and as of now the least performant backend

Additionally, even though ROCM_ATTN supports sinks, it would fall back from the custom attention HIP kernel to a triton implementation, so even though technically the support is there, changing the backend to report that it doesn't have support. Actual triton backends (aiter and unified) are better.

Removing VLLM_ROCM_CUSTOM_PAGED_ATTN. If ROCM_ATTN is selected, the idea is to use this kernel anyway.

As a bonus, fixing the AITER supported condition. AITER is not built for and doesn't support gfx90a

[mergify]

Documentation preview: https://vllm--36702.org.readthedocs.build/en/36702/

[gemini-code-assist]

Code Review

This pull request refactors the attention backend selection for ROCm to prioritize the ROCM_ATTN backend, which is now considered the most performant. It also removes the VLLM_ROCM_CUSTOM_PAGED_ATTN environment variable, simplifying configuration. As part of these changes, ROCM_ATTN now correctly reports that it does not support attention sinks, ensuring that more suitable backends like AITER_UNIFIED are chosen when sinks are required. My review identifies a potential issue with the new backend priority order which may not align with the intended logic.

[mergify]

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

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

[micah-wil]

AMD CI build with this PR to compare against nightly: https://buildkite.com/vllm/amd-ci/builds/5975/steps/canvas
Everything checks out, so this change should be good to merge.

[mergify]

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

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