[Attention] Enable masked MHA for topk sparse attention using FA4

[MatthewBonanni]

Requires vllm-project/flash-attention#127, merge that first

Purpose

For pure prefills, there is a sweet spot of sequence length where a masked MHA pathway is faster than sparse MQA. The DeepSeek V3.2 paper mentions this:

Note that for short-sequence prefilling, we specially implement a masked MHA mode to
simulate DSA, which can achieve higher efficiency under short-context conditions.

This PR implements the two optimizations mentioned in #31473, point 3:

1. Use un-absorbed dense MHA pathway for requests with seq_len <= topk
2. Use un-absorbed masked MHA pathway for requests with seq_len <= threshold (here, threshold is taken as 8192, and we also use a min of 128)

Test Plan

Correctness

pytest tests/v1/attention/test_sparse_mla_backends.py

Performance

python benchmarks/attention_benchmarks/benchmark.py --config benchmarks/attention_benchmarks/configs/mla_sparse_mha_vs_mqa.yaml

Test Result

Correctness

Should pass in CI (V1 Attention)

Performance

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[mergify]

⚠️ The sha of the head commit of this PR conflicts with #34732. Mergify cannot evaluate rules on this PR. ⚠️

[gemini-code-assist]

Code Review

This pull request enables FlashAttention 4 for MLA (Multi-head Latent Attention), particularly for the prefill stage. It introduces a new abstraction layer for different FlashAttention versions, updates the build system to include FA4 components, and adds extensive benchmarking capabilities to compare various prefill backends. The changes also adjust the default prefill backend selection for MLA on Blackwell GPUs. My review focuses on the potential impact of these changes on performance and build stability.

[mergify]

Documentation preview: https://vllm--34744.org.readthedocs.build/en/34744/

[mergify]

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

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

[mergify]

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

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