[OPT] head_dim=256 Triton support — BLOCK=32 for Qwen3.6
CRITICAL DISCOVERY: Qwen3.6-35B-A3B uses head_dim=256 (not 128).
text_cfg.head_dim=256, num_heads=24, num_kv_heads=4, GQA=6
This means ALL previous SMEM calculations were wrong:
BLOCK=64 + head_dim=256: 64×256×2×2 = 64KB > 48KB → OVERFLOW
BLOCK=64 + head_dim=128: 64×128×2×2 = 32KB ≤ 48KB → OK (but wrong model)
Fix: head_dim-dependent BLOCK selection in prefix_prefill.py:
head_dim ≤ 128: BLOCK=64, NUM_WARPS=4 (32KB SMEM)
head_dim = 256: BLOCK=32, NUM_WARPS=4 (32KB SMEM)
head_dim > 256: BLOCK=16, NUM_WARPS=2 (16KB SMEM)
Also: _Q_CHUNK in _run_sdpa_fallback reduced 256→128 for head_dim=256
to avoid OOM on long sequences (256×100K×24×4=2.3GB vs 128×100K×24×4=1.2GB).
Without this patch, Triton prefill CANNOT work for Qwen3.6.
patch_enable_triton.py's try/fallback would always fall back to PyTorch.
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