fix: Fixed CORD dataset and refactored dataset constructors

[fg-mindee]

This PR introduces the following modifications:

• added bounding box clipping to fix negative coordinates on CORD dataset
• fixed dataset docstrings
• refactored dataset constructors

Any feedback is welcome!

[charlesmindee]

Thanks!

[codecov]

Codecov Report

Merging #299 (ea704e9) into main (f6b0ff9) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #299      +/-   ##
==========================================
- Coverage   96.09%   96.04%   -0.06%     
==========================================
  Files          48       48              
  Lines        2124     2122       -2     
==========================================
- Hits         2041     2038       -3     
- Misses         83       84       +1     

Flag	Coverage Δ
unittests	96.04% <100.00%> (-0.06%)	⬇️

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

Impacted Files	Coverage Δ
doctr/datasets/funsd.py	96.87% <ø> (ø)
doctr/datasets/recognition.py	90.00% <ø> (ø)
doctr/datasets/sroie.py	94.73% <ø> (ø)
doctr/datasets/cord.py	97.50% <100.00%> (-0.07%)	⬇️
doctr/datasets/detection.py	97.14% <100.00%> (ø)
doctr/datasets/ocr.py	91.89% <100.00%> (-0.22%)	⬇️
...tr/models/detection/differentiable_binarization.py	93.25% <0.00%> (-0.40%)	⬇️

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