[Eager] eager variable back sync (#44343)

* eager variable back sync

paddle/fluid/eager/eager_tensor.h

@@ -209,6 +209,7 @@ class EagerVariable final {
     if (tensor.defined()) {
       if (tensor.is_dense_tensor()) {
         ConstructVariableFromTensor<phi::DenseTensor>(tensor);
+        src_tensor_ = tensor.impl();
       } else if (tensor.is_selected_rows()) {
         ConstructVariableFromTensor<phi::SelectedRows>(tensor);
       } else if (IsVariableCompatTensor(tensor) &&
@@ -229,6 +230,19 @@ class EagerVariable final {
     }
   }
 
+  ~EagerVariable() {
+    if (src_tensor_) {
+      auto* framework_tensor = var_.GetMutable<phi::DenseTensor>();
+      auto tensor_dense = static_cast<phi::DenseTensor*>(src_tensor_.get());
+      if (framework_tensor->memory_size() > 0 &&
+          (!paddle::platform::is_same_place(framework_tensor->place(),
+                                            tensor_dense->place()) ||
+           framework_tensor->dtype() != tensor_dense->dtype())) {
+        tensor_dense->ShareBufferWith(*framework_tensor);
+      }
+    }
+  }
+
   /** Part 11: Construct paddle::framework::Variable with phi::Tensor **/
   std::shared_ptr<phi::TensorBase> GetTensorBase() {
     // Construct allocation only once.
@@ -304,5 +318,6 @@ class EagerVariable final {
  private:
   std::string name_{""};
   paddle::framework::Variable var_;
+  std::shared_ptr<phi::TensorBase> src_tensor_;
 };
 }  // namespace egr