[Bugfix][SM120][MLA] Validate sparse MLA top-k buffer

[pxljs]

Purpose

Addresses #46726.

This PR improves SM120 FlashInfer sparse MLA construction error handling and adds regression coverage for skip-topk layers that rely on a shared topk_indices_buffer.

The current implementation already supports the valid skip-topk path where indexer is None and the implementation falls back to the shared topk_indices_buffer passed by the upper MLA path. This PR keeps that behavior, and adds targeted tests to make sure it remains covered.

This PR also replaces the internal assertion for the missing top-k buffer case with an explicit ValueError. This makes the failure mode clearer when neither an indexer nor a shared topk_indices_buffer is available.

I checked for duplicate work before opening this PR:

gh issue view 46726 --repo vllm-project/vllm --comments
gh pr list --repo vllm-project/vllm --state open --search "46726 in:body"
gh pr list --repo vllm-project/vllm --state open --search "SM120 sparse MLA topk_indices_buffer"
gh pr list --repo vllm-project/vllm --state open --search "FlashInfer sparse MLA indexer"

Results:

• No open PR directly references [Bug]: sparse-MLA indexer error with GLM 5.2 NVFP4 on RTX 6000 Pro SM120 #46726.
• I did not find an existing open PR that adds this specific SM120 FlashInfer sparse MLA API coverage or replaces the missing top-k buffer assertion with an explicit ValueError.

AI assistance was used while investigating the issue, reasoning about the sparse MLA construction path, and drafting parts of the PR description. I personally reviewed the changed code, verified that the implementation matches the intended behavior, and ran the tests listed below before submitting.

Test Plan

Run the relevant SM120 FlashInfer sparse MLA API tests:

.venv/bin/python -m pytest -s -v tests/v1/attention/test_flashinfer_sparse_mla_sm120_api.py

Run the sparse MLA backend tests:

.venv/bin/python -m pytest -s -v tests/v1/attention/test_sparse_mla_backends.py

Run pre-commit on the modified files:

pre-commit run --files \
  tests/v1/attention/test_flashinfer_sparse_mla_sm120_api.py \
  vllm/v1/attention/backends/mla/flashinfer_mla_sparse_sm120.py

The targeted tests cover:

• Models without index_topk are rejected by the SM120 sparse backend.
• Skip-topk layers with indexer=None can be constructed when a shared topk_indices_buffer is provided.
• A clear ValueError is raised when both indexer and the shared top-k buffer are missing.

Test Result

Passed:

tests/v1/attention/test_flashinfer_sparse_mla_sm120_api.py: all relevant tests passed
tests/v1/attention/test_sparse_mla_backends.py: all relevant tests passed
pre-commit run --files tests/v1/attention/test_flashinfer_sparse_mla_sm120_api.py vllm/v1/attention/backends/mla/flashinfer_mla_sparse_sm120.py: passed

Not fully validated locally:

I do not have access to the original SM120 setup from the issue report, so validation from the issue reporter or maintainers on the original failing environment would be appreciated.

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[claude]

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[mergify]

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

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