refactor: Removed implementations of nn that are now integrated in PyTorch

[frgfm]

This PR introduces the following modifications:

• deprecate SiLU & Mish now that they are integrated in PyTorch (>=1.9.0)
• updated documentation, unittests and readme accordingly
• adapted implementations using those

[codecov]

Codecov Report

Merging #157 (a8b4fdb) into master (735e15a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
+ Coverage   92.25%   92.26%   +0.01%     
==========================================
  Files          52       52              
  Lines        3046     3039       -7     
==========================================
- Hits         2810     2804       -6     
+ Misses        236      235       -1     

Impacted Files	Coverage Δ
holocron/models/darknetv4.py	100.00% <100.00%> (ø)
holocron/models/detection/yolov4.py	97.56% <100.00%> (ø)
holocron/models/rexnet.py	100.00% <100.00%> (ø)
holocron/models/segmentation/unet.py	98.68% <100.00%> (ø)
holocron/nn/functional.py	92.44% <100.00%> (-0.13%)	⬇️
holocron/nn/modules/activation.py	93.33% <100.00%> (-2.13%)	⬇️
holocron/utils/data/collate.py	100.00% <100.00%> (+6.25%)	⬆️