Decode MMA kernel previously used scalar global→shared loads with
LD=HEAD_DIM+8 padding and per-tile scale multiply. This commit brings it
in line with the prefill MMA kernel (which already had these optimizations):
- cp.async K/V loads (bypasses registers, halves load instructions)
- XOR swizzle: LD=HEAD_DIM instead of HEAD_DIM+8 (zero waste smem)
- Pre-scale Q during load (removes per-tile scale multiply in softmax)
- Clean up prefill MMA kernel comments (no code change)
~2x speedup on decode (0.47ms→0.24ms at seq_len=512)
- add case 32 to decode/prefill dispatch switch
- fix swiz_col out-of-bounds for HEAD_DIM=32: XOR mask now limited to chunk count (3 for 32, 7 for >=64) instead of always 7, which produced column offsets >= LD=32 and corrupted shared memory
- restructure decode dispatch to #ifndef/#else/#endif matching prefill
- split astrai/extension/__init__.py into loader.py (kernel .so discovery) and ops.py (wrapper functions + torch SDPA fallback); __init__.py now re-exports the public API
- replace scalar fragment loads with ldmatrix.sync.x4/x2
- add smem row-stride padding (LD = HEAD_DIM + 8) to eliminate 8-way bank conflicts from HEAD_DIM being a 32-bank multiple
- switch build flag from positive to negative: -DASTRAI_NO_MMA for pre-sm_80 only; mma is the default path
- vectorize scalar path smem loads with float4 ld8
- fix pure-C test configs for ld8 alignment
ViperEkura