feat: add aspect ratio for ocr predictor

[charlesmindee]

This PR adds the option to preserve aspect ratio of the pages in the whole OCRPredictor pipeline, which leads to much better results on the detection task.
Any feedback is welcome!

[fg-mindee]

Thanks for the PR!
I added a few comments, could you confirm whether this improves perf with the existing checkpoints? (it isn't specified in the PR details, so I prefer to make sure we use everything)

If that improves for existing checkpoints (trained with images that were stretched, that should strongly indicate that we should update our resizing strategy for detection training asap :))

[charlesmindee]

Yes @fg-mindee it doesn't hurt performances for now, and I will retrain all detection models preserving the aspect ratio to have a coherent pipeline, because it is way better not to deform writtings on documents for the model convergence!

[fg-mindee]

Nice!

[codecov]

Codecov Report

Merging #835 (3c193cb) into main (0310d6c) will decrease coverage by 0.26%.
The diff coverage is 87.40%.

@@            Coverage Diff             @@
##             main     #835      +/-   ##
==========================================
- Coverage   95.99%   95.73%   -0.27%     
==========================================
  Files         131      133       +2     
  Lines        5042     5131      +89     
==========================================
+ Hits         4840     4912      +72     
- Misses        202      219      +17     

Flag	Coverage Δ
unittests	95.73% <87.40%> (-0.27%)	⬇️

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

Impacted Files	Coverage Δ
doctr/models/zoo.py	100.00% <ø> (ø)
doctr/models/predictor/base.py	81.81% <53.84%> (-18.19%)	⬇️
doctr/transforms/modules/pytorch.py	94.52% <84.61%> (-3.79%)	⬇️
doctr/transforms/modules/tensorflow.py	92.90% <90.90%> (-0.16%)	⬇️
doctr/transforms/functional/base.py	97.10% <97.61%> (+3.35%)	⬆️
doctr/io/pdf.py	100.00% <100.00%> (+1.66%)	⬆️
doctr/io/reader.py	100.00% <100.00%> (ø)
doctr/models/predictor/pytorch.py	97.29% <100.00%> (+0.07%)	⬆️
doctr/models/predictor/tensorflow.py	100.00% <100.00%> (ø)
doctr/transforms/functional/pytorch.py	100.00% <100.00%> (ø)
... and 8 more

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[fg-mindee]

Thanks!