Minor Compressions in statements in folder.py

[surgan12]

This PR just compresses two statements into one as mentioned here
#867 (comment)

[codecov-io]

Codecov Report

Merging #874 into master will increase coverage by 0.21%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #874      +/-   ##
==========================================
+ Coverage   55.03%   55.24%   +0.21%     
==========================================
  Files          36       36              
  Lines        3367     3365       -2     
  Branches      552      551       -1     
==========================================
+ Hits         1853     1859       +6     
+ Misses       1377     1372       -5     
+ Partials      137      134       -3

Impacted Files	Coverage Δ
torchvision/datasets/folder.py	76.25% <0%> (+1.85%)	⬆️
torchvision/transforms/transforms.py	82.54% <0%> (+1.29%)	⬆️

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[fmassa]

Thanks!