Async to drop kid

paddle/framework/scope.cc

@@ -17,6 +17,7 @@ limitations under the License. */
 #include <memory>  // for unique_ptr
 #include <mutex>   // for call_once
 #include "glog/logging.h"
+#include "paddle/framework/threadpool.h"
 #include "paddle/string/printf.h"
 
 namespace paddle {
@@ -87,7 +88,8 @@ void Scope::DeleteScope(Scope* scope) {
   auto it = std::find(this->kids_.begin(), this->kids_.end(), scope);
   PADDLE_ENFORCE(it != this->kids_.end(), "Cannot find %p as kid scope", scope);
   this->kids_.erase(it);
-  delete scope;
+  // Make delete async.
+  Async([scope] { delete scope; });
 }
 
 void Scope::Rename(const std::string& origin_name,

paddle/framework/threadpool.h

@@ -159,5 +159,14 @@ class ThreadPool {
   std::condition_variable completed_;
 };
 
+// Run a function asynchronously.
+// NOTE: The function must return void. If the function need to return a value,
+// you can use lambda to capture a value pointer.
+template <typename Callback, typename ARGS>
+std::future<void> Async(Callback callback, ARGS... args) {
+  return ThreadPool::GetInstance()->Run(
+      [&] { callback(std::forward(args)...); });
+};
+
 }  // namespace framework
 }  // namespace paddle