add PADDLE_ENFORCE for dygraph to optimize error throw (#19783)

* add PADDLE_ENFORCE for dygraph to optimize error throw, test=develop

* fix some error, test=develop

* delete PADDLE_ENFORCE_EQ in VarBase::NewVarBase, test=develop

paddle/fluid/imperative/tracer.cc

@@ -230,10 +230,12 @@ void Tracer::TraceBackward(const std::shared_ptr<OpBase>& fwd_op,
               VLOG(3) << op->Type();
             }
           }
+          auto grad_name = (*(fwd_var_iter->second))->GradVarBase()->Name();
           if (!grad_pending_ops.empty()) {
             for (const auto& op : grad_pending_ops) {
-              PADDLE_ENFORCE_NOT_NULL(op,
-                                      "No nullptr should be grad_pending op");
+              PADDLE_ENFORCE_NOT_NULL(
+                  op, "No nullptr should be grad_pending op for variable %s ",
+                  grad_name);
               if (visited_preceding_ops.count(op) == 0) {
                 visited_preceding_ops.insert(op);
                 grad_op->InsertGradPendingOps(op);