Remove V0 attention backends

[WoosukKwon]

Summary

• restrict CUDA and ROCm attention backend selection to the V1 engine and raise errors when a removed V0 backend is requested
• update runtime helpers and tests to consume the V1 attention metadata/backends and add local ALiBi utilities for kernel tests
• skip V0-only model initialization coverage and drop the legacy attention selector tests that depended on V0 backends

Testing

• pytest tests/kernels/attention/test_prefix_prefill.py::test_contexted_kv_attention --maxfail=1 (fails: missing tblib dependency)

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https://chatgpt.com/codex/tasks/task_b_68d02a781064832dacffd35e5f979636

[gemini-code-assist]

Code Review

This pull request undertakes a significant refactoring to remove the legacy V0 attention backends, streamlining the codebase to exclusively use V1 backends. The changes are comprehensive, involving the deletion of numerous V0 backend files, updating tests to align with V1 backends, and modifying platform-specific code to raise errors for requests of removed backends. This consolidation simplifies the attention backend infrastructure. The changes appear consistent and well-executed, with the addition of RuntimeError for V0 backend requests providing a clear failure mode for this breaking change.