Supports Color augmentation.

[v-qjqs]

This PR implements color augmentations which only transforms image (e.g. color, equalize, sharpness) in mmcv.

[codecov]

Codecov Report

Merging #542 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #542      +/-   ##
==========================================
+ Coverage   64.58%   64.63%   +0.05%     
==========================================
  Files         131      131              
  Lines        7502     7513      +11     
  Branches     1327     1330       +3     
==========================================
+ Hits         4845     4856      +11     
  Misses       2411     2411              
  Partials      246      246              

Flag	Coverage Δ
#unittests	64.63% <100.00%> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
mmcv/image/__init__.py	100.00% <100.00%> (ø)
mmcv/image/photometric.py	100.00% <100.00%> (ø)
mmcv/video/io.py	91.41% <100.00%> (+0.05%)	⬆️

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