!5765 modify the log which nodes forming loop

Merge pull request !5765 from Margaret_wangrui/get_loop_nodes_log

mindspore/ccsrc/backend/session/kernel_graph.cc

@@ -286,6 +286,9 @@ void KernelGraph::GetLoopNodesByDFS(AnfNodePtr node, uint32_t *loop_num) {
     MS_LOG(DEBUG) << "Node [" << node->DebugString() << "] don't have input edges.";
     return;
   }
+  if (*loop_num != 0) {
+    return;
+  }
   visited_nodes_.insert(node);
   for (auto input_edge : node_input_edges_[node]) {
     size_t input_num = node_input_num_[input_edge.first];
@@ -300,19 +303,24 @@ void KernelGraph::GetLoopNodesByDFS(AnfNodePtr node, uint32_t *loop_num) {
       AnfNodePtr node_iter = node;
       MS_EXCEPTION_IF_NULL(node_iter);
       MS_LOG(DEBUG) << "Print loop nodes start:";
-      for (; node_iter != input_edge.first; node_iter = edge_to_[node_iter]) {
-        MS_EXCEPTION_IF_NULL(node_iter);
+      for (; node_iter != input_edge.first && node_iter != nullptr; node_iter = edge_to_[node_iter]) {
         loop_nodes_.push(node_iter);
         node_input_num_[node_iter]--;
         MS_LOG(DEBUG) << "Get loop node:" << node_iter->DebugString();
       }
-      loop_nodes_.push(node_iter);
-      loop_nodes_.push(node);
-      (*loop_num)++;
-      node_input_num_[node_iter]--;
-      MS_LOG(DEBUG) << "Get loop node:" << node_iter->DebugString();
-      MS_LOG(DEBUG) << "Get loop node:" << node->DebugString();
-      MS_LOG(DEBUG) << "Print loop nodes end, Loop num:" << *loop_num;
+      if (node_iter != nullptr) {
+        loop_nodes_.push(node_iter);
+        loop_nodes_.push(node);
+        (*loop_num)++;
+        node_input_num_[node_iter]--;
+        MS_LOG(DEBUG) << "Get loop node:" << node_iter->DebugString();
+        MS_LOG(DEBUG) << "Get loop node:" << node->DebugString();
+        MS_LOG(DEBUG) << "Print loop nodes end, Loop num:" << *loop_num;
+        while (!loop_nodes_.empty()) {
+          loop_nodes_.pop();
+        }
+        return;
+      }
     }
   }
 }