fix code bug in CPU compilation

test=develop

paddle/fluid/framework/parallel_executor.cc

@@ -97,8 +97,8 @@ std::unique_ptr<ir::Graph> ParallelExecutorPrivate::PrepareGCAndRefCnts(
     if (gcs_.count(place) > 0) {
       continue;
     }
-#ifdef PADDLE_WITH_CUDA
     GarbageCollector<Tensor> *gc = nullptr;
+#ifdef PADDLE_WITH_CUDA
     if (platform::is_gpu_place(place)) {
       if (IsFastEagerDeletionModeEnabled()) {
         gc = new UnsafeFastGPUGarbageCollector<Tensor>(