[Features] Add census loss

[MeowZheng]

Motivation

as title

Modification

1. add mmflow/models/losses/census_loss.py
2. modify mmflow/models/losses/init.py
3. modify tests/test_models/test_losses.py

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.51%. Comparing base (b33aa32) to head (3a0e015).
Report is 52 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
- Coverage   68.53%   68.51%   -0.03%     
==========================================
  Files          88       89       +1     
  Lines        4968     4999      +31     
  Branches      713      713              
==========================================
+ Hits         3405     3425      +20     
- Misses       1453     1464      +11     
  Partials      110      110              

Flag	Coverage Δ
unittests	68.51% <100.00%> (-0.03%)	⬇️

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