datasets: fix benchmark dataset folder

[ocramz]

When running stack bench I get

bench: /Users/ocramz/.cache/datasets-hs/cifar-10-imagefolder/Truck: getDirectoryContents:openDirStream: does not exist (No such file or directory)

I guess it's a matter of copying the test data in a temporary directory before these tests.

[stites]

Ah! There was a provisioning step on my machine to run the benchmark. I'll try to get to this, but the rough idea was to stream cifar-10 into the designated .cache/datasets-hs/cifar-10-imagefolder in the standard image-folder style (documented in pytorch's ImageFolder class, or the keras ImageGenerator.flow_from_directory method). I think you can do this in ghci with the current infrastructure in datasets, but I don't recall how to do it off the top of my head.