feat: Improved Darknet and YOLO flexibility

[frgfm]

This PR adds user flexibility in Darknet and YOLO usage by:

• adding the possibility to specify the normalization layer of Darknets
• added backbone freezing option, which will also convert backbone BN to frozen BN

[codecov]

Codecov Report

Merging #45 into master will decrease coverage by 0.04%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage   84.50%   84.46%   -0.05%     
==========================================
  Files          29       29              
  Lines        1407     1416       +9     
==========================================
+ Hits         1189     1196       +7     
- Misses        218      220       +2     

Impacted Files	Coverage Δ
holocron/models/darknet.py	98.12% <93.10%> (-1.22%)	⬇️
holocron/models/detection/yolo.py	98.04% <100.00%> (+<0.01%)	⬆️