refactor topological_sort to use Node.get_users() (#2536)

* use Node.get_users() to replace deprecated func

* use const ref instead of auto

* used const auto& in graph_util.cpp

src/ngraph/graph_util.cpp

@@ -185,7 +185,7 @@ bool ngraph::is_post_dominated(Node* X, Node* Y)
         stack.pop_front();
         if (curr != Y)
         {
-            for (auto next : curr->get_users())
+            for (const auto& next : curr->get_users())
             {
                 if (visited.count(next.get()) == 0)
                 {
@@ -458,7 +458,7 @@ NodeVector ngraph::get_subgraph_outputs(const NodeVector& nodes,
             continue;
         }
 
-        for (auto u : n->get_users())
+        for (const auto& u : n->get_users())
         {
             if (nodes_set.count(u) == 0 && (!ignore_unused || is_used(u.get())))
             {
@@ -487,7 +487,7 @@ bool ngraph::is_used(Node* node)
             instances_seen.insert(n);
         }
         stack.pop_front();
-        for (auto arg : n->get_users())
+        for (const auto& arg : n->get_users())
         {
             if (instances_seen.count(arg.get()) == 0)
             {
@@ -501,7 +501,7 @@ bool ngraph::is_used(Node* node)
 size_t ngraph::get_user_count(Node* node)
 {
     size_t count = 0;
-    for (auto node_user : node->get_users())
+    for (const auto& node_user : node->get_users())
     {
         count += is_used(node_user.get());
     }

src/ngraph/graph_util.hpp

@@ -96,17 +96,11 @@ namespace ngraph
             result_list.push_back(node_map[independent_node]);
             independent_nodes.pop_front();
 
-            for (auto& output : independent_node->get_outputs())
+            for (const std::shared_ptr<Node>& user : independent_node->get_users())
             {
-                for (auto& input : output.get_inputs())
+                if (--node_dependency_count[user.get()] == 0)
                 {
-                    auto user = input->get_raw_pointer_node();
-                    node_dependency_count[user] -= 1;
-                    size_t count = node_dependency_count[user];
-                    if (count == 0)
-                    {
-                        independent_nodes.push_back(user);
-                    }
+                    independent_nodes.push_back(user.get());
                 }
             }
 
@@ -179,17 +173,11 @@ namespace ngraph
             result_list.push_back(node_map[independent_node]);
             independent_nodes.pop_front();
 
-            for (auto& output : independent_node->get_outputs())
+            for (const std::shared_ptr<Node>& user : independent_node->get_users())
             {
-                for (auto& input : output.get_inputs())
+                if (--node_dependency_count[user.get()] == 0)
                 {
-                    auto user = input->get_raw_pointer_node();
-                    node_dependency_count[user] -= 1;
-                    size_t count = node_dependency_count[user];
-                    if (count == 0)
-                    {
-                        independent_nodes.push_back(user);
-                    }
+                    independent_nodes.push_back(user.get());
                 }
             }