Rename KineticsVideo to Kinetics400 + docs

[fmassa]

Make it explicit that this was done with Kinetics400 in mind.

Also add docs.

cc @bjuncek

[bjuncek]

Looks good!

[codecov-io]

Codecov Report

Merging #1183 into master will increase coverage by 0.06%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1183      +/-   ##
==========================================
+ Coverage   65.82%   65.89%   +0.06%     
==========================================
  Files          79       79              
  Lines        5849     5849              
  Branches      890      890              
==========================================
+ Hits         3850     3854       +4     
+ Misses       1728     1726       -2     
+ Partials      271      269       -2

Impacted Files	Coverage Δ
torchvision/datasets/__init__.py	100% <100%> (ø)	⬆️
torchvision/datasets/kinetics.py	34.78% <50%> (ø)	⬆️
torchvision/transforms/functional.py	71.38% <0%> (+0.28%)	⬆️
torchvision/transforms/transforms.py	81.53% <0%> (+0.58%)	⬆️

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