feat: Added possibility to load model from HF Hub

[frgfm]

This PR introduces the following modifications:

• moves all classification models to models.classification
• updates copyright years on models
• adds model_from_hf_hub function to instantiate and load a pretrained model from the hub
• updates reqs
• adds unittest

[codecov]

Codecov Report

Merging #198 (b32801a) into master (af8f22e) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
+ Coverage   93.49%   93.51%   +0.02%     
==========================================
  Files          55       56       +1     
  Lines        3088     3100      +12     
==========================================
+ Hits         2887     2899      +12     
  Misses        201      201              

Impacted Files	Coverage Δ
holocron/models/segmentation/unet3p.py	98.21% <ø> (ø)
holocron/models/segmentation/unetpp.py	100.00% <ø> (ø)
holocron/models/__init__.py	100.00% <100.00%> (ø)
holocron/models/classification/__init__.py	100.00% <100.00%> (ø)
holocron/models/classification/darknet.py	100.00% <100.00%> (ø)
holocron/models/classification/darknetv2.py	100.00% <100.00%> (ø)
holocron/models/classification/darknetv3.py	85.24% <100.00%> (ø)
holocron/models/classification/darknetv4.py	100.00% <100.00%> (ø)
holocron/models/classification/pyconvresnet.py	100.00% <100.00%> (ø)
holocron/models/classification/repvgg.py	88.50% <100.00%> (ø)
... and 10 more