[gemma4_31b][cuda][tq4] split-K decode: size BLOCK_M to num_groups (not fixed 64)

The split-K decode launcher hardcoded BLOCK_M=64 for the pack-GQA path, but
only L_q*num_groups rows are valid (=8 for Gemma4 global, H_Q=32/H_KV=4). That
left 56/64 M-rows idle while the QK/PV tensor-core MMAs still computed all 64,
and made acc[64, HEAD_DIM=512] fp32 = 128 KB/CTA, spilling registers to local
memory. Size BLOCK_M = max(16, next_pow2(L_q*num_groups)) instead (16 for
Gemma4 global), removing the wasted MMA rows and the spills. Decode-only;
prefill path unchanged.

Author: Gasoonjia

backends/cuda/triton/kernels/tq4_sdpa.py

@@ -1201,7 +1201,15 @@ def _launch_tq4_decode_splitk(
 
     if pack_gqa:
         H_grid = H_KV
-        BLOCK_M = 64  # Pack GQA uses larger BLOCK_M for efficiency
+        # Size BLOCK_M to the actually-packed rows (L_q * num_groups; for decode
+        # L_q=1 so == num_groups), rounded up to the bf16 tensor-core MMA minimum
+        # of 16 -- instead of a fixed 64. Gemma4 global has num_groups=8, so the
+        # old BLOCK_M=64 left 56/64 M-rows idle (the QK/PV MMAs still computed all
+        # 64 rows) AND made acc[BLOCK_M, HEAD_DIM] fp32 = 64*512*4 = 128 KB/CTA,
+        # which blows past the register file and spills to local memory. Matching
+        # BLOCK_M to num_groups removes both the wasted MMA rows and the spills.
+        _packed_m = L_Q * num_groups
+        BLOCK_M = max(16, 1 << (_packed_m - 1).bit_length())
     else:
         H_grid = H_Q
         BLOCK_M = 32