Expose docs for io and ops package

[fmassa]

I had do modify the docstrings to use Napoleon NumPy style, because Napoleon Google Style doesn't support multiple return arguments.

[codecov-io]

Codecov Report

Merging #1189 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #1189     +/-   ##
=========================================
+ Coverage   65.64%   65.74%   +0.1%     
=========================================
  Files          79       79             
  Lines        5827     5827             
  Branches      889      889             
=========================================
+ Hits         3825     3831      +6     
+ Misses       1731     1728      -3     
+ Partials      271      268      -3

Impacted Files	Coverage Δ
torchvision/io/video.py	72% <ø> (ø)	⬆️
torchvision/ops/boxes.py	94.73% <ø> (ø)	⬆️
torchvision/transforms/transforms.py	81.13% <0%> (+0.19%)	⬆️
torchvision/transforms/functional.py	71.38% <0%> (+1.44%)	⬆️

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