metal : wrap each operation in debug group (ggml/690)

ggml-metal.m

@@ -1067,6 +1067,8 @@ bool ggml_metal_graph_compute(
                     GGML_ASSERT(!"unsupported op");
                 }
 
+                [encoder pushDebugGroup:[NSString stringWithCString:ggml_op_desc(dst)]];
+
                 const int64_t  ne00 = src0 ? src0->ne[0] : 0;
                 const int64_t  ne01 = src0 ? src0->ne[1] : 0;
                 const int64_t  ne02 = src0 ? src0->ne[2] : 0;
@@ -2423,6 +2425,8 @@ bool ggml_metal_graph_compute(
                             GGML_ASSERT(false);
                         }
                 }
+
+                [encoder popDebugGroup];
             }
 
             if (encoder != nil) {