[Feature] Support random video compression during training

[ckkelvinchan]

Motivation

To support random video compression during training, for RealBasicVSR.

Modifications

1. Modify random_degradaions.py to support video compression
2. Modify build.yml to install av in CI

[codecov]

Codecov Report

Merging #646 (6339792) into master (4075d92) will increase coverage by 0.05%.
The diff coverage is 93.87%.

@@            Coverage Diff             @@
##           master     #646      +/-   ##
==========================================
+ Coverage   80.41%   80.47%   +0.05%     
==========================================
  Files         198      198              
  Lines       11169    11218      +49     
  Branches     1780     1788       +8     
==========================================
+ Hits         8982     9028      +46     
- Misses       1913     1915       +2     
- Partials      274      275       +1     

Flag	Coverage Δ
unittests	80.45% <93.87%> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
mmedit/datasets/pipelines/__init__.py	100.00% <ø> (ø)
mmedit/datasets/pipelines/random_degradations.py	93.89% <93.87%> (-0.01%)	⬇️

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