Add SSN losses.

[JackyTown]

This PR adds SSN losses which contain OHEMHingeLoss, activity_loss, completeness_loss, classwise_regression_loss.

[codecov]

Codecov Report

Merging #52 into master will increase coverage by 0.42%.
The diff coverage is 93.20%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   84.90%   85.32%   +0.42%     
==========================================
  Files          73       75       +2     
  Lines        3889     4083     +194     
  Branches      632      652      +20     
==========================================
+ Hits         3302     3484     +182     
- Misses        485      493       +8     
- Partials      102      106       +4     

Flag	Coverage Δ
#unittests	85.32% <93.20%> (+0.42%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmaction/datasets/pipelines/__init__.py	100.00% <ø> (ø)
mmaction/models/__init__.py	100.00% <ø> (ø)
mmaction/models/losses/ohem_hinge_loss.py	75.75% <75.75%> (ø)
mmaction/datasets/pipelines/formating.py	93.85% <85.71%> (+0.34%)	⬆️
mmaction/datasets/rawframe_dataset.py	87.36% <95.23%> (+2.00%)	⬆️
mmaction/datasets/pipelines/loading.py	93.79% <96.62%> (+0.70%)	⬆️
mmaction/models/losses/ssn_loss.py	98.14% <98.14%> (ø)
mmaction/models/losses/__init__.py	100.00% <100.00%> (ø)
... and 1 more

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