fix bug in grad_op compute for dygraph, test=develop (#20975)

paddle/fluid/imperative/tracer.cc

@@ -20,12 +20,6 @@
 namespace paddle {
 namespace imperative {
 
-struct OpBaseCmp {
-  bool operator()(OpBase* first, OpBase* second) {
-    return first->id() > second->id();
-  }
-};
-
 static std::vector<std::unique_ptr<OpBase>> CreateGradOpBases(
     const OpBase* fw_op_base, const NameVarBaseMap& in,
     const NameVarBaseMap& out) {
@@ -130,7 +124,7 @@ void Tracer::TraceBackward(const std::shared_ptr<OpBase>& fwd_op,
       }
     }
 
-    std::set<OpBase*, OpBaseCmp> visited_preceding_ops;
+    std::set<OpBase*> visited_preceding_ops;
     for (auto& grad_out_it : grad_out) {
       bool flag_clear_list = false;
       for (auto& var_base_it : grad_out_it.second) {