fix unit test

paddle/fluid/operators/detail/grpc_server_test.cc

@@ -121,13 +121,10 @@ TEST(PREFETCH, DISABLED_CPU) {
   std::string in_var_name("ids");
   std::string out_var_name("out");
 
-  detail::RPCClient::GetInstance();
+  auto client = detail::RPCClient::GetInstance();
-
+  client->AsyncPrefetchVariable("127.0.0.1:8889", ctx, scope, in_var_name,
-  // detail::RPCClient::GetInstance();
+                                out_var_name);
-  // client->Wait();
+  client->Wait();
-  // client->AsyncPrefetchVariable("127.0.0.1:8889", ctx, scope, in_var_name,
-  //                             out_var_name);
-  // client->Wait();
 
   auto var = scope.Var(out_var_name);
   auto value = var->GetMutable<framework::SelectedRows>()->value();