feat: Added DropBlock implementation

[frgfm]

This PR introduces dropblock by:

• adding functional and module APIs
• adding them to the documentation
• adding corresponding unittests

[codecov]

Codecov Report

Merging #53 into master will increase coverage by 0.22%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   85.23%   85.45%   +0.22%     
==========================================
  Files          30       31       +1     
  Lines        1571     1602      +31     
==========================================
+ Hits         1339     1369      +30     
- Misses        232      233       +1     

Impacted Files	Coverage Δ
holocron/nn/modules/dropblock.py	94.11% <94.11%> (ø)
holocron/nn/functional.py	95.41% <100.00%> (+0.62%)	⬆️
holocron/nn/modules/__init__.py	100.00% <100.00%> (ø)