Added support for VisionDataset

[pmeier]

This adds support for the 'new' VisionDataset superclass for datasets which were added after the original PR #749.

[codecov-io]

Codecov Report

Merging #838 into master will increase coverage by 0.54%.
The diff coverage is 54.54%.

@@            Coverage Diff             @@
##           master     #838      +/-   ##
==========================================
+ Coverage   53.92%   54.47%   +0.54%     
==========================================
  Files          36       36              
  Lines        3347     3306      -41     
  Branches      549      543       -6     
==========================================
- Hits         1805     1801       -4     
+ Misses       1409     1372      -37     
  Partials      133      133

Impacted Files	Coverage Δ
torchvision/datasets/imagenet.py	22.68% <50%> (+1.47%)	⬆️
torchvision/datasets/celeba.py	16.09% <50%> (+1.35%)	⬆️
torchvision/datasets/sbd.py	31.66% <50%> (+0.08%)	⬆️
torchvision/datasets/caltech.py	19% <62.5%> (-2.14%)	⬇️
torchvision/transforms/transforms.py	83.52% <0%> (+0.67%)	⬆️

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[fmassa]

Awesome, thanks!