Hello @ptrblck,
I am Sorry, I didnt answer your this question as I am new in deep learning.
What are you passing as args.gpus?
It is like this:
model = torch.nn.DataParallel(model, device_ids=args.gpus).cuda()
cudnn.benchmark = True
I have also tried it like device_ids=[0,1,2,3]  but still I have assertion error.