Fix BatchProviderROI incorrect super constructor invokation

[0wu]

(1) so the BatchProviderROI inherited fields can be properly saved by torch.save
(2) The following code produce correct output

require('nnf')
bp1 = nnf.BatchProviderROI({dataset="1"})
print(bp1.dataset) --prints 1
bp2 = nnf.BatchProviderROI({dataset="2"})
print(bp1.dataset) --prints 2 (should be 1)

[fmassa]

Thanks!
I'm refactoring all the code so that it's more modular/extensible, it's in branch refactoring. I'll add your changes there as well.