Further expose VisionDataset

[pmeier]

Follow up to #876 as requested.

[fmassa]

Thanks!

[codecov-io]

Codecov Report

Merging #878 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #878      +/-   ##
==========================================
+ Coverage    55.3%   55.39%   +0.08%     
==========================================
  Files          36       36              
  Lines        3376     3376              
  Branches      553      553              
==========================================
+ Hits         1867     1870       +3     
+ Misses       1372     1370       -2     
+ Partials      137      136       -1

Impacted Files	Coverage Δ
torchvision/datasets/__init__.py	100% <ø> (ø)	⬆️
torchvision/transforms/transforms.py	82.54% <0%> (+0.64%)	⬆️

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