feat: Added support of ReXNet

[frgfm]

This PR introduces ReXNet models for image classification by:

• simplifying ResNet architecture layout
• adapting the implementation of https://github.com/clovaai/rexnet of ReXNet
• adding corresponding unittests
• adding it to the documentation
• updating Readme

[codecov]

Codecov Report

Merging #56 into master will increase coverage by 0.44%.
The diff coverage is 97.50%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   85.88%   86.33%   +0.44%     
==========================================
  Files          32       32              
  Lines        1714     1785      +71     
==========================================
+ Hits         1472     1541      +69     
- Misses        242      244       +2     

Impacted Files	Coverage Δ
holocron/models/resnet.py	93.71% <97.50%> (+2.36%)	⬆️