feat: Added FP16 support to doctr.datasets

[fg-mindee]

Following up on #263, this PR introduces the following modifications:

• switched default image dtype of TF datasets to FP32 (before it was tf.uint8)
• added FP16 mode on all datasets
• added corresponding unittests
• updated references script to reflect changes (no need to perform 255 division manually anymore)

Please note that there is a breaking change here, the default image dtype of datasets in TF has been switched from tf.uint8 (between 0 and 255) to tf.float32 (between 0 and 1).

Any feedback is welcome!

[codecov]

Codecov Report

Merging #367 (e4ac24d) into main (c18025b) will increase coverage by 0.02%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##             main     #367      +/-   ##
==========================================
+ Coverage   96.18%   96.20%   +0.02%     
==========================================
  Files          83       83              
  Lines        3461     3480      +19     
==========================================
+ Hits         3329     3348      +19     
  Misses        132      132              

Flag	Coverage Δ
unittests	96.20% <91.66%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
doctr/datasets/ocr.py	91.89% <80.00%> (+0.22%)	⬆️
doctr/datasets/funsd.py	96.87% <83.33%> (+0.10%)	⬆️
doctr/datasets/cord.py	97.50% <85.71%> (+0.13%)	⬆️
doctr/datasets/sroie.py	94.87% <87.50%> (+0.27%)	⬆️
doctr/datasets/datasets/base.py	95.34% <100.00%> (+0.11%)	⬆️
doctr/datasets/datasets/pytorch.py	100.00% <100.00%> (ø)
doctr/datasets/datasets/tensorflow.py	100.00% <100.00%> (ø)
doctr/datasets/detection.py	96.96% <100.00%> (+0.09%)	⬆️
doctr/datasets/recognition.py	89.47% <100.00%> (ø)

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[charlesmindee]

Thanks!