Supports brightness and contrast augmentations

[v-qjqs]

This PR implements brightness and contrast augmentations in mmcv.

[codecov]

Codecov Report

Merging #546 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #546      +/-   ##
==========================================
+ Coverage   64.71%   64.77%   +0.05%     
==========================================
  Files         133      133              
  Lines        7570     7582      +12     
  Branches     1334     1334              
==========================================
+ Hits         4899     4911      +12     
  Misses       2424     2424              
  Partials      247      247              

Flag	Coverage Δ
#unittests	64.77% <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)

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