Fix a recently introduced race condition in DataLayer

src/caffe/layers/base_data_layer.cu

@@ -20,7 +20,9 @@ void BasePrefetchingDataLayer<Dtype>::Forward_gpu(
     caffe_copy(batch->label_.count(), batch->label_.gpu_data(),
         top[1]->mutable_gpu_data());
   }
-
+  // Ensure the copy is synchronous wrt the host, so that the next batch isn't
+  // copied in meanwhile.
+  CUDA_CHECK(cudaStreamSynchronize(cudaStreamDefault));
   prefetch_free_.push(batch);
 }