message: add sycn for multi_cross_layer_test
diff: @@ -194,6 +194,7 @@ class MultiCrossLayerTest {
  void gpu_fprop_() {
  layer_->fprop(true);
  + CK_CUDA_THROW_(cudaDeviceSynchronize());
  return;
  }
  @@ -222,6 +223,7 @@ class MultiCrossLayerTest {
  void gpu_bprop_() {
  layer_->bprop();
  + CK_CUDA_THROW_(cudaDeviceSynchronize());
  return;
  }
  