move root_scope->DropKids() into Finalize() so that we do not have to drop all the kids

test=develop

paddle/fluid/framework/dist_multi_trainer.cc

@@ -73,6 +73,7 @@ void DistMultiTrainer::Finalize() {
   }
   pull_dense_worker_->Stop();
   dataset_ptr_->DestroyReaders();
+  root_scope_->DropKids();
 }
 
 }  // end namespace framework

paddle/fluid/framework/executor.cc

@@ -143,8 +143,6 @@ void Executor::RunFromDataset(const ProgramDesc& main_program, Scope* scope,
   trainer->Run();
   VLOG(3) << "Trainer going to finalize";
   trainer->Finalize();
-  VLOG(3) << "Drop current scope kids";
-  scope->DropKids();
   return;
 }
 

paddle/fluid/framework/multi_trainer.cc

@@ -76,6 +76,7 @@ void MultiTrainer::Finalize() {
     th.join();
   }
   dataset_ptr_->DestroyReaders();
+  root_scope_->DropKids();
 }
 
 }  // end namespace framework