[Feat] Add Layout model LW-DETR

[felixdittrich92]

This PR:

• Add LW-DETR layout detection model
• Add corresponding tests

[felixdittrich92]

@simont2k A first draft not yet done - If you want it's ready to have a first look - I will split the PR into several PRs to make the review easier

TODOs:

• Fix mypy
• Further clean-up / minor improvements
• Add missing documentation

[codecov]

Codecov Report

❌ Patch coverage is 97.54768% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.71%. Comparing base (ec4e076) to head (fc8fb63).

Files with missing lines	Patch %	Lines
doctr/models/layout/lw_detr/pytorch.py	97.79%	7 Missing ⚠️
doctr/models/layout/lw_detr/base.py	96.15%	4 Missing ⚠️
doctr/models/layout/lw_detr/layers/pytorch.py	98.39%	4 Missing ⚠️
doctr/models/layout/zoo.py	92.00%	2 Missing ⚠️
doctr/models/layout/predictor/pytorch.py	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2059      +/-   ##
==========================================
+ Coverage   96.43%   96.71%   +0.27%     
==========================================
  Files         147      156       +9     
  Lines        6486     7220     +734     
==========================================
+ Hits         6255     6983     +728     
- Misses        231      237       +6     

Flag	Coverage Δ
unittests	96.71% <97.54%> (+0.27%)	⬆️

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