fix error message of Inplace strategy (#30520)

paddle/fluid/imperative/basic_engine.cc

@@ -221,13 +221,16 @@ void BasicEngine::PrepareGradAccumulators(
 void BasicEngine::PrepareDeps() {
   PADDLE_ENFORCE_EQ(
       node_deps_.empty(), true,
-      platform::errors::AlreadyExists("Op deps must be initialized."));
-  PADDLE_ENFORCE_EQ(
-      accumulators_.empty(), true,
-      platform::errors::AlreadyExists("Accumulators must be initialized."));
-  PADDLE_ENFORCE_EQ(
-      accumulators_with_grad_node_.empty(), true,
-      platform::errors::AlreadyExists("Accumulators must be initialized."));
+      platform::errors::AlreadyExists("Op deps are not empty before preparing "
+                                      "it for backward network execution."));
+  PADDLE_ENFORCE_EQ(accumulators_.empty(), true,
+                    platform::errors::AlreadyExists(
+                        "Accumulators are not empty before preparing it for "
+                        "backward network execution."));
+  PADDLE_ENFORCE_EQ(accumulators_with_grad_node_.empty(), true,
+                    platform::errors::AlreadyExists(
+                        "Accumulators with grad_node as the key are not empty "
+                        "before preparing it for backward network execution."));
 
   std::queue<GradOpNode*> q;
   std::unordered_set<GradOpNode*> visited;

paddle/fluid/imperative/variable_wrapper.h

@@ -258,11 +258,13 @@ class VariableWrapper {
 
     auto shared_node = grad_node_.lock();
     if (shared_node != grad_node) {
+      if (grad_node->InplaceGradNameMap().empty()) {
+        // grad_node doesn't have Inplace message
         PADDLE_ENFORCE_EQ(
-          !shared_node || !grad_node->InplaceGradNameMap().empty(), true,
+            shared_node, nullptr,
             platform::errors::PermissionDenied(
                 "Cannot set gradient op twice unless using Inplace Strategy."));
-      if (shared_node) {
+      } else if (shared_node) {
         VLOG(3) << "The gradient op of Var (" << Name()
                 << ") has been set twice. Because Inplace Strategy is used.";
       }