Generalize the unified attention pipeline to support NumWarpGroups=1
(single warp group) with a serial K/V loop, in addition to the existing
NumWarpGroups=2 interleaved pipeline.
New decode kernel traits use 4 warps (sequence<4,1,1>) with kBlockM=128
and kBlockQ=16 for GQA-8, reducing Q tile padding waste from 31/32 to
15/16 for decode workloads (max_seqlen_q=1).
Host-side dispatch (is_decode_shape) routes low-token workloads to the
decode kernel automatically.
Benchmark results on d64 GQA-8 (via aiter):
- 64-seq decode: 2.2x slower -> 1.27x slower (1.73x speedup)
- 512-seq decode: 3.5x slower -> 1.6x slower (2.2x speedup)
- 1-seq decode: 0.83x (CK wins) -> 0.81x (no regression)
- Prefill: unchanged (uses original 8-warp kernel)
Made-with: Cursor
583b017321