after deleting a Net instance, the GPU memory is not released

This is my construction of Classifier:
Classifier::Classifier(const std::string& model_file,
const std::string& trained_file)
{
#ifdef CPU_ONLY
Caffe::set_mode(Caffe::CPU);
#else
Caffe::set_mode(Caffe::GPU);
#endif

/* Load the network. */
net_.reset(new Net(model_file, TEST));
net_->CopyTrainedLayersFrom(trained_file);

CHECK_EQ(net_->num_inputs(), 1) << "Network should have exactly one input.";
CHECK_EQ(net_->num_outputs(), 1) << "Network should have exactly one output.";

Blob* input_layer = net_->input_blobs()[0];
num_channels_ = input_layer->channels();
CHECK(num_channels_ == 3 || num_channels_ == 1)
<< "Input layer should have 1 or 3 channels.";
input_geometry_ = cv::Size(input_layer->width(), input_layer->height());

}
and,this is my test code:
Classifier* cnn_1 = new Classifier(lenet_model_file, lenet_trained_file);
delete cnn_1;
the problem is after deleting cnn_1, the memory of GPU is not released.
Please tell me how to release the memory of GPU caused by loading the model.