feat: Added implementation of Pyramidal convolution

[frgfm]

This PR adds support of pyramidal convolutions by:

• adding PyConv2d implementation
• updating unittests
• updating documentation
• updating README

[codecov]

Codecov Report

Merging #66 into master will increase coverage by 0.13%.
The diff coverage is 94.52%.

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   88.80%   88.93%   +0.13%     
==========================================
  Files          34       35       +1     
  Lines        1759     1817      +58     
==========================================
+ Hits         1562     1616      +54     
- Misses        197      201       +4     

Impacted Files	Coverage Δ
holocron/models/res2net.py	92.72% <ø> (ø)
holocron/models/pyconvresnet.py	93.33% <93.33%> (ø)
holocron/models/resnet.py	94.11% <95.23%> (-0.53%)	⬇️
holocron/nn/modules/conv.py	98.68% <95.23%> (-1.32%)	⬇️
holocron/models/__init__.py	100.00% <100.00%> (ø)