Add some error messages for the op without double grads. (#25951) (#25963)

* Add some error messages for the op without double grads.

* fix the test_imperative_double_grad UT.

paddle/fluid/imperative/partial_grad_engine.cc

@@ -870,13 +870,15 @@ void PartialGradTask::RunEachOp(OpBase *op) {
   if (create_graph_) {
     auto double_grad_node = CreateGradOpNode(op->InnerOp(), tmp_ins, tmp_outs,
                                              op->Attrs(), op->place());
-    if (double_grad_node) {
+    PADDLE_ENFORCE_NOT_NULL(
+        double_grad_node,
+        platform::errors::NotFound("The Op %s doesn't have any grad op.",
+                                   op->Type()));
     VLOG(10) << "Create " << double_grad_node->size()
              << " double grad op(s) for " << op->Type()
              << ", pending ops: " << GradPendingOpTypes(*double_grad_node);
     double_grad_nodes_.emplace_back(std::move(double_grad_node));
   }
-  }
 
   VLOG(10) << "There are " << grads_to_accumulate_.size() << " to sum gradient";