feat: Added implementation of PolyLoss

[frgfm]

This PR introduces the following modifications:

• adds functional and module implementations of PolyLoss
• adds it to the documentation and the README
• added dedicated unittests

[codecov]

Codecov Report

Merging #209 (fa69f17) into master (f0f6379) will decrease coverage by 0.01%.
The diff coverage is 92.00%.

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
- Coverage   93.51%   93.50%   -0.02%     
==========================================
  Files          56       56              
  Lines        3100     3125      +25     
==========================================
+ Hits         2899     2922      +23     
- Misses        201      203       +2     

Impacted Files	Coverage Δ
holocron/nn/modules/loss.py	90.36% <87.50%> (-0.31%)	⬇️
holocron/nn/functional.py	92.30% <94.11%> (+0.18%)	⬆️