@ptrblck, thanks much for your response. You asked about my GPU:
  In: torch.cuda.get_device_name()
  Out: GeForce GT 710

Found this link 6 to supported Cuda products; the GT 710 is not listed. Yet, the product box claims Cuda support, nvidia-smi gives the info listed earlier and the Nvidia UI claims it has 192 Cuda cores.
Maybe I was a bit too cheap in getting the lowest-cost GPU that supports both a 4K screen and (supposedly) Cuda…
So, the first step is to verify that the GT 710 does support Cuda. Is there a non-PyTorch way to do that?
If that checks out, I can try upgrading to the just-released 460 driver. (Upgrading Nvidia drivers on Linux Mint is a nightmare.)