avc-test-model does not release GPU memory after testing

[csy530216]

For example, I found that avc-test-model DIRECTORY --gpu X does not release the memory of GPU X after testing; ./avc-test-model-against-attack [...] --model_gpu X --attack_gpu Y does not release the memory of GPU X after testing, either. To release the memory, manually killing the process is needed. I think this is an important thing when using lab-shared servers.