[datasets] unify np_dtype and fix comments

[felixdittrich92]

This PR:

• unify np_dtype usage instead of np.float32
• fix wrong comments or add missing
  @fg-mindee
• fix wrong shape in SynthText if ndim != 3

[fg-mindee]

Thanks for the PR!
I agree, however I think the comment about polygons should be more specific. Perhaps something like:

# (x, y) coordinates of top left, top right, bottom right, bottom left corners

[codecov]

Codecov Report

Merging #782 (1f0f5ca) into main (e5ca2d5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #782   +/-   ##
=======================================
  Coverage   95.99%   95.99%           
=======================================
  Files         130      130           
  Lines        4870     4873    +3     
=======================================
+ Hits         4675     4678    +3     
  Misses        195      195           

Flag	Coverage Δ
unittests	95.99% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
doctr/datasets/doc_artefacts.py	93.93% <ø> (ø)
doctr/datasets/ic03.py	97.36% <ø> (ø)
doctr/datasets/ic13.py	96.15% <ø> (ø)
doctr/datasets/iiit5k.py	96.77% <ø> (ø)
doctr/datasets/ocr.py	92.30% <ø> (ø)
doctr/datasets/sroie.py	96.87% <ø> (ø)
doctr/datasets/svhn.py	94.73% <ø> (ø)
doctr/datasets/svt.py	97.22% <ø> (ø)
doctr/datasets/cord.py	97.43% <100.00%> (+0.06%)	⬆️
doctr/datasets/detection.py	95.65% <100.00%> (+0.19%)	⬆️
... and 3 more

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[fg-mindee]

Looks good to me! Only a question about a transpose that you changed 👌

[fg-mindee]

Thanks Felix!