test: Switched to pytest suite

[frgfm]

This PR introduces the following modifications:

• switched the entire test suite from unittest to pytest
• fixed complement_cross_entropy in place ops
• removed OneCycle since it has been integrated into PyTorch
• revamped README
• updated CONTRIBUTING
• updated CI job & requirements

[codecov]

Codecov Report

Merging #131 (76dc4ec) into master (cb4b906) will increase coverage by 0.55%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #131      +/-   ##
==========================================
+ Coverage   91.03%   91.59%   +0.55%     
==========================================
  Files          51       50       -1     
  Lines        2935     2866      -69     
==========================================
- Hits         2672     2625      -47     
+ Misses        263      241      -22     

Impacted Files	Coverage Δ
holocron/optim/__init__.py	100.00% <ø> (ø)
holocron/nn/functional.py	92.57% <75.00%> (-3.97%)	⬇️
holocron/optim/lars.py	63.63% <0.00%> (-1.82%)	⬇️
holocron/optim/lamb.py	81.81% <0.00%> (-1.82%)	⬇️
holocron/optim/ralars.py	80.59% <0.00%> (-1.50%)	⬇️
holocron/models/detection/yolov4.py	97.54% <0.00%> (-0.50%)	⬇️
holocron/nn/modules/activation.py	95.45% <0.00%> (ø)
holocron/nn/modules/loss.py	93.75% <0.00%> (+1.25%)	⬆️