Feature/free kid scope (PaddlePaddle#4951)

* Delete kid

* Delete local scope

Author: reyoung

paddle/framework/executor.cc

@@ -84,8 +84,7 @@ void Executor::Run(const ProgramDesc& pdesc, Scope* scope, int block_id) {
     op->Run(local_scope, *device);
   }
 
-  // TODO(tonyyang-svail):
-  //  - Destroy local_scope
+  scope->DeleteScope(&local_scope);
 }
 
 }  // namespace framework

paddle/framework/scope.cc

@@ -65,6 +65,13 @@ void Scope::DropKids() {
   kids_.clear();
 }
 
+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;
+}
+
 framework::Scope& GetGlobalScope() {
   static framework::Scope* g_scope = nullptr;
   if (g_scope == nullptr) {

paddle/framework/scope.h

@@ -59,6 +59,8 @@ class Scope {
   /// Find the scope or an ancestor scope that contains the given variable.
   const Scope* FindScope(const Variable* var) const;
 
+  void DeleteScope(Scope* scope);
+
   /// Drop all kids scopes belonged to this scope.
   void DropKids();