llama.cpp Hexagon flash_attn 改按 KV head 分核，PP 最高提升 58%

hexagon: head-parallel flash_attn partitioning for row-split multicore In row-split mode each core computes its output row shard of every MUL_MAT, but flash_attn was previously partitioning by Q tokens (flat qrow split) instead of by heads. This forced every core to read the full KV cache (all n_kv_heads), negating the memory bandwidth benefit of multicore on flash_attn. Change both HMX and HVX flash_attn kernels to partition by KV heads when n_kv_heads is divisible by n_cores: core i processes heads [i*n_kv_heads/N, (i+1)*n_kv_heads/N) exclusively, reading only its head shard of the KV cache. Falls back to the original token-block split when n_kv_heads % n_cores != 0 (e.g. Gemma-4 with 2 KV heads on 4 cores). Controlled by GGML_HEXAGON_FA_HEAD_SPLIT (default 1 = on). The flag is packed into bit 1 of the existing is_dst_fp32 kparams byte to stay within the 128-byte kernel_params blob limit. Measured gains at 4c row-split (PP t/s, ubatch=1024): Qwen3-0.6B: 6977 -> 11026 (+58%) llama-3.2-3B: 3717 -> 5522 (+49%) Qwen3.5-4B: 2739 -> 2855 (+4%) Gemma-4 MoE: no change (MoE FFN dominates, fallback path) TG is unchanged (flash_attn is a small fraction of decode time relative to the matmul+barrier cost per layer). hex-fa: cleanup kern_params and head-split selection hex-fa: add -fa-head-split option to run.py hex-mdev: update matmul solver to account for reduced work in row-split scenarios hex-mmid: better work splitting by expers in multi-dev scenarios hex-fa: update HMX gating based on the model/n-hvx/ctx-len sweep hex-fa: precompute softcap/scale on the host hexagon: flatten matmul into 2d to use HMX in multi-sequence hex-mm: cleanup kparams and use collapse to 3/4D -> 2D mapping hex-mm: fix typo in collapse fallback hex-mm: another pass at consistent naming for act tensors hex-mm: add support for colapsing dims in fused matmuls hex-build: fix WoS build errors hex-mm: make sure to enforce dst stride in can_collapse hex-fa: add a onliner commit for head-split check hex-fa: remove unused local head_split var hex-fa: tighten up can_split checks hex-mm: update unfused paths to use act instead src1 hex-mm: make sure to check all dsts for splitting hexagon: fix the second weight chunk address in the batched HMX matmul prologue hexagon: F16 activation and ragged N in the HMX matmul hex-mm: tighten the ragged/split checks in mdev cases hex-mm: enable MM fusion for F16 activations hex-mm: pass tiled sizes to the solver in fused paths hex-mmid: remove scalar divs from expert mapping loops hex-mmid: proper cacheline safety enforcement for mdev splits hex-mm: improve solver for mdev split scanarios and tail handling hex-mm: remove redundant checks hex-mm: fix fused HMX MUL_MAT_NX drops the final partial tile for quantized weights hex-mm: better handling of ragged shapes (removes scalar memset of vtcm) Co-authored-by: ebateni ebateni@qti.qualcomm.com Co-authored-by: Jhen-Jie Hong iainst0409@gmail.com Co-authored-by: Yiwei Shao yiwei@aizip.ai