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

test=develop

paddle/fluid/platform/lodtensor_printer_test.cc

@@ -21,4 +21,5 @@ TEST(LodTensorPrinter, PrintVar) {
   paddle::platform::PrintVar(&scope, "NotAVar", "We don't have var");
   paddle::framework::Variable* v = scope.Var("NotAVar");
   paddle::platform::PrintVar(&scope, "NotAVar", "Now we have a var");
+  v->Clear();
 }