feat: Added custom weight decay for normalization layers

[frgfm]

This PR introduces the following modifications:

• adds an option in trainer methods to specify normalization layers' weight decay
• updates unittests
• adds the possibility in training scripts
• updates mypy config

Closes #107

[codecov]

Codecov Report

Merging #162 (670317b) into master (ee903c4) will decrease coverage by 0.05%.
The diff coverage is 95.12%.

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
- Coverage   93.96%   93.91%   -0.06%     
==========================================
  Files          51       51              
  Lines        2981     3005      +24     
==========================================
+ Hits         2801     2822      +21     
- Misses        180      183       +3     

Impacted Files	Coverage Δ
holocron/trainer/utils.py	95.00% <90.47%> (-5.00%)	⬇️
holocron/trainer/core.py	90.99% <100.00%> (-0.40%)	⬇️
holocron/optim/wrapper.py	85.93% <0.00%> (+0.78%)	⬆️