increment loading index regardless of CUDA

[pmeier]

Fixes #206. Although the installation instructions install CUDA and a compatible PyTorch version

https://github.com/libffcv/ffcv/blob/f25386557e213711cc8601833add36ff966b80b2/README.md?plain=1#L38

I didn't find a hint or the like that CUDA is required. The presence of a flag like

https://github.com/libffcv/ffcv/blob/f25386557e213711cc8601833add36ff966b80b2/ffcv/loader/epoch_iterator.py#L16

also supports that.