Miscellaneous dataset fixes

[pmeier]

Fixes some argument checking for the LSUN and STL10 dataset as discussed in #1167.

[codecov-io]

Codecov Report

Merging #1174 into master will decrease coverage by 0.14%.
The diff coverage is 7.89%.

@@            Coverage Diff             @@
##           master    #1174      +/-   ##
==========================================
- Coverage   65.91%   65.76%   -0.15%     
==========================================
  Files          79       79              
  Lines        5809     5825      +16     
  Branches      882      883       +1     
==========================================
+ Hits         3829     3831       +2     
- Misses       1711     1725      +14     
  Partials      269      269

Impacted Files	Coverage Δ
torchvision/datasets/lsun.py	22.91% <10%> (+0.44%)	⬆️
torchvision/datasets/stl10.py	26.26% <5.55%> (-1.52%)	⬇️
torchvision/transforms/transforms.py	80.94% <0%> (-0.2%)	⬇️

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[fmassa]

LGTM, thanks!

(but having tests would be even better!)

[pmeier]

(but having tests would be even better!)

I went for the low-hanging fruit first ;)