Add docs to UCF101 and HMDB51 datasets

[fmassa]

Also add the transform argument to the datasets.

cc @bjuncek

[codecov-io]

Codecov Report

Merging #1184 into master will decrease coverage by 0.34%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1184      +/-   ##
==========================================
- Coverage   65.87%   65.53%   -0.35%     
==========================================
  Files          79       79              
  Lines        5849     5837      -12     
  Branches      890      891       +1     
==========================================
- Hits         3853     3825      -28     
- Misses       1726     1741      +15     
- Partials      270      271       +1

Impacted Files	Coverage Δ
torchvision/datasets/ucf101.py	25% <0%> (-3.21%)	⬇️
torchvision/datasets/hmdb51.py	27.65% <0%> (-3.3%)	⬇️
torchvision/datasets/video_utils.py	84.07% <0%> (-1.12%)	⬇️
torchvision/transforms/transforms.py	80.55% <0%> (-0.99%)	⬇️
torchvision/transforms/functional.py	70.23% <0%> (-0.87%)	⬇️

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