add some comment for added NodeTopologicalOrder

lite/core/mir/subgraph/subgraph_detector.cc

@@ -314,6 +314,11 @@ void SubgraphDetector::InitNodes(node_map_t *nodes) {
 std::vector<std::vector<Node *>> SubgraphDetector::ExtractSubgraphs(
     node_map_t *nodes) {
   for (auto &n_tpo : graph_->NodeTopologicalOrder()) {
+    // different orders when traversing nodes in graph may lead to
+    // different subgraph division, which may generate different result
+    // with device such as MLU. These different results are all "right"
+    // but a little confusing. Thus the topological order is used instead
+    // of the address of the node in graph.
     CHECK(nodes->find(n_tpo) != nodes->end());
     node_dat_t *node = (*nodes)[n_tpo];
     if (!node->marked) {