Use joint transform in Cityscapes

[TheCodez]

This adds a joint transform to Citscapes as started in #872

[codecov-io]

Codecov Report

Merging #1024 into master will increase coverage by 0.16%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1024      +/-   ##
==========================================
+ Coverage   63.29%   63.46%   +0.16%     
==========================================
  Files          65       65              
  Lines        5152     5154       +2     
  Branches      772      772              
==========================================
+ Hits         3261     3271      +10     
+ Misses       1667     1661       -6     
+ Partials      224      222       -2

Impacted Files	Coverage Δ
torchvision/datasets/cityscapes.py	20% <0%> (+0.55%)	⬆️
torchvision/datasets/mnist.py	51.18% <0%> (+0.47%)	⬆️
torchvision/transforms/transforms.py	82.54% <0%> (+0.64%)	⬆️
torchvision/transforms/functional.py	72.15% <0%> (+0.94%)	⬆️

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[fmassa]

Thanks for the PR!

Can you add transforms to the documentation as well?

Otherwise it looks good to me!

[fmassa]

Thanks!