Test videos with B-Frames

[fmassa]

Also extend video saving to support different codecs and options. Notably, we can now save with lossless compression

[codecov-io]

Codecov Report

Merging #1157 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1157      +/-   ##
==========================================
- Coverage   65.65%   65.61%   -0.05%     
==========================================
  Files          70       70              
  Lines        5527     5529       +2     
  Branches      853      853              
==========================================
- Hits         3629     3628       -1     
- Misses       1639     1641       +2     
- Partials      259      260       +1

Impacted Files	Coverage Δ
torchvision/io/video.py	73.95% <100%> (+2.68%)	⬆️
torchvision/transforms/transforms.py	80.55% <0%> (-0.99%)	⬇️

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