VGGFace2 dataset

@fmassa how would you like to go about this?

Currently, I preprocess VGGFace2 by cropping out the face from each image using the provided bounding box csv. After doing that, I load it as an ImageFolder dataset.

VGGFace2 also has a bunch of other CSVs that annotate gender, age group, and facial landmarks.

I propose we allow users to specify the targets they want (such as ID, gender, age, bounding box etc.) and have a crop_bb=False if they only want the face.

What do you think?