Make the new video API available through torchvision.io

[bjuncek]

Make the new video API available through torchvision.io for easier usage.
Test are updated as well.

[codecov]

Codecov Report

Merging #2773 into master will increase coverage by 0.01%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #2773      +/-   ##
==========================================
+ Coverage   73.16%   73.17%   +0.01%     
==========================================
  Files          96       96              
  Lines        8398     8403       +5     
  Branches     1315     1316       +1     
==========================================
+ Hits         6144     6149       +5     
  Misses       1855     1855              
  Partials      399      399              

Impacted Files	Coverage Δ
torchvision/io/_video_opt.py	39.37% <0.00%> (ø)
torchvision/io/__init__.py	100.00% <100.00%> (ø)

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[fmassa]

Subsumed by #2778