Standardize str argument verification in datasets

[pmeier]

First steps towards #1132. I left some TODOs and FIXME for review / discussion.

[codecov-io]

Codecov Report

Merging #1167 into master will increase coverage by 0.56%.
The diff coverage is 52.38%.

@@            Coverage Diff             @@
##           master    #1167      +/-   ##
==========================================
+ Coverage   65.11%   65.68%   +0.56%     
==========================================
  Files          72       72              
  Lines        5619     5607      -12     
  Branches      867      857      -10     
==========================================
+ Hits         3659     3683      +24     
+ Misses       1699     1668      -31     
+ Partials      261      256       -5

Impacted Files	Coverage Δ
torchvision/datasets/imagenet.py	90.9% <100%> (+3.4%)	⬆️
torchvision/datasets/cityscapes.py	80% <100%> (+10%)	⬆️
torchvision/datasets/svhn.py	68.75% <100%> (+2.75%)	⬆️
torchvision/datasets/caltech.py	21.83% <25%> (+0.24%)	⬆️
torchvision/datasets/mnist.py	51.7% <33.33%> (+0.98%)	⬆️
torchvision/datasets/voc.py	23.07% <33.33%> (+0.97%)	⬆️
torchvision/datasets/celeba.py	19.73% <33.33%> (+1.66%)	⬆️
torchvision/datasets/sbd.py	33.33% <33.33%> (+2.08%)	⬆️
torchvision/datasets/stl10.py	27.77% <50%> (+0.6%)	⬆️
torchvision/datasets/utils.py	60.64% <68.42%> (+1.08%)	⬆️
... and 3 more

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[fmassa]

Generally looks very good, thanks!

I'm approving it, but most of the TODOs (that I have confirmed) should go before merging.

Also, having tests for more datasets would make reviewing dataset PRs a lot easier!

[fmassa]

Test failure is unrelated and test_randomresized_params seems flaky, I opened an issue in #1170

[pmeier]

Should be done now.

Also, having tests for more datasets would make reviewing dataset PRs a lot easier!

Agreed. If I find some time, I try to add some more.

[fmassa]

Thanks!