Enhance the error message when GrapOpMaker is null. (#19070)

* Enhance the error message when GrapOpMaker is null.
test=develop

* Call Proto() instead of directly using proto_ pointer.
test=develop

* Rollback to use proto_ directly, because some sepecial grad ops, such some double grad ops, donot have proto.
test=develop

paddle/fluid/framework/op_info.h

@@ -52,21 +52,29 @@ struct OpInfo {
   }
 
   const proto::OpProto& Proto() const {
-    PADDLE_ENFORCE_NOT_NULL(proto_, "Operator Proto has not been registered");
+    PADDLE_ENFORCE_NOT_NULL(proto_, "Operator's Proto has not been registered");
     PADDLE_ENFORCE(proto_->IsInitialized(),
-                   "Operator Proto must be initialized in op info");
+                   "Operator's Proto must be initialized in op info");
     return *proto_;
   }
 
   const OpCreator& Creator() const {
     PADDLE_ENFORCE_NOT_NULL(creator_,
-                            "Operator Creator has not been registered");
+                            "Operator's Creator has not been registered");
     return creator_;
   }
 
   const GradOpMakerFN& GradOpMaker() const {
-    PADDLE_ENFORCE_NOT_NULL(grad_op_maker_,
-                            "Operator GradOpMaker has not been registered.");
+    // Normally, proto_ should not be null, except some special operators, such
+    // as LeaklyReluDoubleGrad op.
+    std::string type = proto_ ? proto_->type() : "unknown";
+    PADDLE_ENFORCE_NOT_NULL(
+        grad_op_maker_,
+        "Operator %s's GradOpMaker has not been "
+        "registered.\nPlease check whether %s_op has "
+        "grad_op.\nIf not, please set stop_gradient to True "
+        "for its input and output variables using var.stop_gradient=True.",
+        type.c_str(), type.c_str());
     return grad_op_maker_;
   }