ggml : add GGML_DEFAULT_N_THREADS

ggml.c

@@ -9363,7 +9363,7 @@ struct ggml_cgraph ggml_build_forward(struct ggml_tensor * tensor) {
     struct ggml_cgraph result = {
         /*.n_nodes      =*/ 0,
         /*.n_leafs      =*/ 0,
-        /*.n_threads    =*/ 0,
+        /*.n_threads    =*/ GGML_DEFAULT_N_THREADS,
         /*.work_size    =*/ 0,
         /*.work         =*/ NULL,
         /*.nodes        =*/ { NULL },

ggml.h

@@ -182,6 +182,7 @@ extern "C" {
 #define GGML_MAX_PARAMS        16
 #define GGML_MAX_CONTEXTS      64
 #define GGML_MAX_OPT           4
+#define GGML_DEFAULT_N_THREADS 4
 
 #ifdef __ARM_NEON
 // we use the built-in 16-bit float type