[Feature] Add clamp activation layer.

[nbei]

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[codecov]

Codecov Report

Merging #685 (2a0d628) into master (eaf25af) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #685      +/-   ##
==========================================
+ Coverage   64.79%   64.84%   +0.04%     
==========================================
  Files         135      135              
  Lines        7835     7845      +10     
  Branches     1388     1389       +1     
==========================================
+ Hits         5077     5087      +10     
  Misses       2493     2493              
  Partials      265      265              

Flag	Coverage Δ
unittests	64.84% <100.00%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
mmcv/cnn/bricks/activation.py	100.00% <100.00%> (ø)

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