I was able to call torch.cuda.is_available() and torch.device() (not strictly a CUDA function, I think, but was using it to set the device to “cuda:0”) without anything blowing up.
@peterjc123 , I’d uninstalled pytorch + torchvision + pytorch-nightly before attempting the (re)install operation.  Note that I’m using conda, not pip, in case that has anything to do with the issue.  To get torchvision (re)installed on top of pytorch-nightly, I had to use the --no-deps flag since it otherwise requires pytorch to be install.
It appears I don’t actually have a PYTHONPATH environment variable on this system (FWIW - installed using the Anaconda graphical installer); I don’t see pytorch referenced in the normal PATH variable either.