Add class weight

[kennymckormick]

1. Add class_weight for CrossEntropyLoss and BCELossWithLogits
2. switch mmcv to 1.2.4 in CI

[codecov]

Codecov Report

Merging #509 (8a28890) into master (e40df88) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #509      +/-   ##
==========================================
+ Coverage   84.73%   84.76%   +0.03%     
==========================================
  Files         120      120              
  Lines        8407     8424      +17     
  Branches     1373     1377       +4     
==========================================
+ Hits         7124     7141      +17     
  Misses        935      935              
  Partials      348      348              

Flag	Coverage Δ
unittests	84.75% <100.00%> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
mmaction/models/losses/cross_entropy_loss.py	100.00% <100.00%> (ø)

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