feat: add rotated ckpts for pytorch DBNet + fix line resolution for rotated pages

[charlesmindee]

This PR adds rotated ckpts in the db_resnet50 cfg in torch, and fixes the document builder:

• fixes the angle computation to straighten boxes when resolving lines
• fixes the order of points in the rotated polygons (rectify_loc_preds function)
• fixes the line resolution

to be merged after #731
Any feedback is welcome!

[codecov]

Codecov Report

Merging #743 (2d16432) into main (f2f1f21) will decrease coverage by 0.08%.
The diff coverage is 91.42%.

@@            Coverage Diff             @@
##             main     #743      +/-   ##
==========================================
- Coverage   96.19%   96.10%   -0.09%     
==========================================
  Files         129      129              
  Lines        4810     4805       -5     
==========================================
- Hits         4627     4618       -9     
- Misses        183      187       +4     

Flag	Coverage Δ
unittests	96.10% <91.42%> (-0.09%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
doctr/datasets/detection.py	93.75% <ø> (ø)
doctr/models/_utils.py	97.59% <ø> (-1.21%)	⬇️
...s/detection/differentiable_binarization/pytorch.py	97.60% <66.66%> (-0.78%)	⬇️
doctr/models/detection/zoo.py	96.00% <80.00%> (-4.00%)	⬇️
doctr/utils/visualization.py	90.26% <80.00%> (-0.73%)	⬇️
doctr/io/elements.py	92.09% <100.00%> (ø)
doctr/models/builder.py	99.01% <100.00%> (-0.12%)	⬇️
...dels/detection/differentiable_binarization/base.py	93.49% <100.00%> (ø)
doctr/utils/geometry.py	98.73% <100.00%> (ø)
... and 1 more

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[fg-mindee]

Thanks a lot! Minor improvement suggestions in the review 👍

[fg-mindee]

Thanks, only minor changes left! Still some CI jobs failing though

[fg-mindee]

Thank a lot for all this work!