feat: Added a learning rate finder

[frgfm]

This PR aims at:

• adding a lr_finder function in the utils module
• adding a corresponding unittest
• adding it to the documentation

[codecov]

Codecov Report

Merging #26 into master will decrease coverage by 0.53%.
The diff coverage is 91.17%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
- Coverage   71.56%   71.02%   -0.54%     
==========================================
  Files          22       24       +2     
  Lines        1143     1177      +34     
==========================================
+ Hits          818      836      +18     
- Misses        325      341      +16     

Impacted Files	Coverage Δ
holocron/utils/misc.py	90.90% <90.90%> (ø)
holocron/utils/__init__.py	100.00% <100.00%> (ø)
holocron/models/detection/yolo.py	90.68% <0.00%> (-8.08%)	⬇️