feat: Added LR finder option in PyTorch training scripts

[fg-mindee]

Following up on #478, this PR adds a LR finder feature in all PyTorch training scripts

Here is how it renders for artefact detection:

Any feedback is welcome!

[codecov]

Codecov Report

Merging #703 (533e0a4) into main (d67abd4) will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #703      +/-   ##
==========================================
+ Coverage   96.01%   96.14%   +0.12%     
==========================================
  Files         124      124              
  Lines        4648     4648              
==========================================
+ Hits         4463     4469       +6     
+ Misses        185      179       -6     

Flag	Coverage Δ
unittests	96.14% <ø> (+0.12%)	⬆️

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Impacted Files	Coverage Δ
...dels/detection/differentiable_binarization/base.py	90.55% <0.00%> (+1.11%)	⬆️
doctr/models/detection/linknet/base.py	89.10% <0.00%> (+3.96%)	⬆️

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[charlesmindee]

Thanks!