ggml-webgpu: only use subgroup-matrix path when head dims are divisible by sg_mat_k / sg_mat_n (ggml-org#23020)

Author: ArberSephirotheca

ggml/src/ggml-webgpu/ggml-webgpu-shader-lib.hpp

@@ -777,15 +777,18 @@ inline ggml_webgpu_flash_attn_decisions ggml_webgpu_flash_attn_get_decisions(
     const bool tile_can_dispatch_all_q_rows =
         context.max_subgroup_size > 0 &&
         context.max_wg_size >= GGML_WEBGPU_FLASH_ATTN_TILE_Q_TILE * context.max_subgroup_size;
-    const bool use_tile = context.supports_subgroups && !context.supports_subgroup_matrix && K->type == GGML_TYPE_F16 &&
+    const bool use_subgroup_matrix =
+        context.supports_subgroup_matrix && context.sg_mat_k > 0 && context.sg_mat_n > 0 &&
+        context.src0->ne[0] % context.sg_mat_k == 0 && context.src2->ne[0] % context.sg_mat_n == 0;
+    const bool use_tile = context.supports_subgroups && !use_subgroup_matrix && K->type == GGML_TYPE_F16 &&
                           V->type == GGML_TYPE_F16 && f16_vec4_aligned &&
                           (context.src0->ne[0] % GGML_WEBGPU_FLASH_ATTN_TILE_KV_VEC_WIDTH == 0) &&
                           (context.src2->ne[0] % GGML_WEBGPU_FLASH_ATTN_TILE_KV_VEC_WIDTH == 0) &&
                           tile_can_dispatch_all_q_rows && !use_vec;
 
     decisions.path = use_vec                          ? GGML_WEBGPU_FLASH_ATTN_PATH_VEC :
                      use_tile                         ? GGML_WEBGPU_FLASH_ATTN_PATH_TILE :
-                     context.supports_subgroup_matrix ? GGML_WEBGPU_FLASH_ATTN_PATH_SUBGROUP_MATRIX :
+                     use_subgroup_matrix              ? GGML_WEBGPU_FLASH_ATTN_PATH_SUBGROUP_MATRIX :
                                                         GGML_WEBGPU_FLASH_ATTN_PATH_NONE;
 
     if (decisions.path == GGML_WEBGPU_FLASH_ATTN_PATH_NONE) {