Move cifar10 download script to scripts/datasets and mention if loading fails.
Fix import.
Fix path joining.
Add note for LISA lab people to skip download step.
Fix typos.
Let function() do the downcasting, if needed.

But still emit warnings to user. I think using allow_input_downcast=True
is a more elegant solition than casting explicitly. Also, this won't break
GPU execution on double precision, if supported in future.

Thanks for the hint, Ian!