added support for running demos on CPU when no CUDA devices are found

[glhr]

The current demo implementation gives an AssertionError when no NVidia driver is found. I replaced .cuda() with .to(device=torch.device('cuda' if torch.cuda.is_available() else 'cpu')) so that the image and webcam demos can be run on CPU when no CUDA device is found.

[HanClinto]

Works great, thank you!!