[Eager] polish general_grad (#47151)

paddle/fluid/eager/general_grad.h

@@ -165,9 +165,6 @@ class GeneralGrad {
   void GetGraphInfoBetweenTargets(const std::deque<GradNodeBase*>& init_queue) {
     VLOG(6) << "Runing In GetGraphInfoBetweenTargets";
 
-    // Calculate in_degree for each node
-    std::unordered_map<GradNodeBase*, int> node_in_degree_map;
-
     // Copy nodes
     std::deque<GradNodeBase*> queue = init_queue;
     std::unordered_set<GradNodeBase*> visited;
@@ -196,12 +193,6 @@ class GeneralGrad {
           // Or it could also originated from dispensable inputs
           if (!next_node) continue;
 
-          // Update in_degree
-          if (!node_in_degree_map.count(next_node)) {
-            node_in_degree_map[next_node] = 0;
-          }
-          node_in_degree_map[next_node]++;
-
           // Record depending relationship
           (depending_nodes_)[next_node].emplace(node);
           queue.push_back(next_node);