Added 2:4 sparsity to skip softmax method

[rohansjoshi]

Summary

Adds an apply_sparse24: bool config option to the existing flash_skip_softmax method. When enabled, a 2:4 structured sparsity mask (top-2 of every 4 elements along seq_k) is AND-ed with the skip-softmax block mask in
both prefill and decode phases.

This is a pure PyTorch-level feature for research and analysis — not a performance optimization. It allows studying the interaction between block-level and 2:4 structured sparsity patterns.

Changes

• config.py — New apply_sparse24 field on SparseAttentionAttributeConfig; new SPARSE24_SKIP_SOFTMAX and SPARSE24_SKIP_SOFTMAX_CALIB preset configs.
• methods/flash_skip_softmax.py — Reads the flag and applies the 2:4 mask inside calc_correction_factor_and_p.
• hf_sa.py — Exposes --sparse_attn sparse24_skip_softmax and --sparse_attn sparse24_skip_softmax_calib as CLI choices.

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[codecov]

Codecov Report

❌ Patch coverage is 34.78261% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.22%. Comparing base (fe83270) to head (6afa360).

Files with missing lines	Patch %	Lines
...y/attention_sparsity/methods/flash_skip_softmax.py	21.05%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1019      +/-   ##
==========================================
- Coverage   70.25%   70.22%   -0.04%     
==========================================
  Files         220      220              
  Lines       25368    25391      +23     
==========================================
+ Hits        17822    17830       +8     
- Misses       7546     7561      +15     

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