resolve onnx Split conversion with empty split attribute

tools/onnx/onnx2ncnn.cpp

@@ -2072,15 +2072,25 @@ int main(int argc, char** argv)
         {
             int axis = get_node_attr_i(node, "axis", 0);
             std::vector<int> split = get_node_attr_ai(node, "split");
-            if (axis < 1 || split.size() < 2)
-                fprintf(stderr, "Unsupported split attributes !\n");
+            if (axis < 1)
+                fprintf(stderr, "Unsupported split axis !\n");
 
             fprintf(pp, " -23300=%d", output_size);
-            for (int i=0; i< split.size() - 1; i++)
+            if (split.empty())
             {
-                fprintf(pp, ",%d", split[i]);
+                for (int i=0; i<output_size; i++)
+                {
+                    fprintf(pp, ",-233");
+                }
+            }
+            else
+            {
+                for (int i=0; i<split.size() - 1; i++)
+                {
+                    fprintf(pp, ",%d", split[i]);
+                }
+                fprintf(pp, ",-233");
             }
-            fprintf(pp, ",-233");
             fprintf(pp, " 1=%d", axis - 1);
         }
         else if (op == "Sqrt")