metal : Fix dimension constraint violation in matmul2d descriptor (ggml-org#21048)

Updates Metal tensor API test probe to fix the dimension constraint violation in the matmul2d descriptor (at least one value must be a multiple of 16).

Author: lathrys-at

ggml/src/ggml-metal/ggml-metal-device.m

@@ -690,7 +690,7 @@ ggml_metal_device_t ggml_metal_device_init(int device) {
                     "    auto tB = B.slice((int)tgid.x, 0); \n"
                     " \n"
                     "    matmul2d< \n"
-                    "        matmul2d_descriptor(8, 8, dynamic_extent), \n"
+                    "        matmul2d_descriptor(16, 16, dynamic_extent), \n"
                     "        execution_simdgroups<4>> mm; \n"
                     " \n"
                     "    auto cT = mm.get_destination_cooperative_tensor<decltype(tA), decltype(tB), float>(); \n"
@@ -740,7 +740,7 @@ ggml_metal_device_t ggml_metal_device_init(int device) {
                     "    auto tB = B.slice((int)tgid.x, 0); \n"
                     " \n"
                     "    matmul2d< \n"
-                    "        matmul2d_descriptor(8, 8, dynamic_extent), \n"
+                    "        matmul2d_descriptor(16, 16, dynamic_extent), \n"
                     "        execution_simdgroups<4>> mm; \n"
                     " \n"
                     "    auto cT = mm.get_destination_cooperative_tensor<decltype(tA), decltype(tB), float>(); \n"