feat: Adds GaussianBlur, random font for CharGenerator and improves training scripts

[fg-mindee]

This PR introduces the following modifications:

• adds an implementation of GaussianBlur for TensorFlow using tf addons
• adds the possibility to pick a random font in a list for CharGenerator
• improved synthesize_txt_img to automatically resolve the font size and image size depending on the text
• updates character classification training scripts to add more augmentations
• fixed the validation set of char classification (by default, without color inversion, the images had a black background and text in white all the time, I switched the split to 90% of white background + dark text)

This PR was used to retrain all PyTorch classification models successfully, and TensorFlow are ongoing.

Any feedback is welcome!

[charlesmindee]

Thanks for the PR!

[codecov]

Codecov Report

Merging #758 (20bf498) into main (106a1e1) will decrease coverage by 0.22%.
The diff coverage is 78.26%.

@@            Coverage Diff             @@
##             main     #758      +/-   ##
==========================================
- Coverage   96.20%   95.98%   -0.23%     
==========================================
  Files         129      129              
  Lines        4799     4829      +30     
==========================================
+ Hits         4617     4635      +18     
- Misses        182      194      +12     

Flag	Coverage Δ
unittests	95.98% <78.26%> (-0.23%)	⬇️

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

Impacted Files	Coverage Δ
doctr/datasets/classification/pytorch.py	100.00% <ø> (ø)
doctr/datasets/classification/tensorflow.py	100.00% <ø> (ø)
doctr/datasets/classification/base.py	85.41% <76.66%> (-8.34%)	⬇️
doctr/transforms/modules/tensorflow.py	92.17% <81.25%> (-1.89%)	⬇️
doctr/models/_utils.py	95.60% <0.00%> (-3.30%)	⬇️
doctr/transforms/modules/base.py	94.44% <0.00%> (-1.39%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 106a1e1...20bf498. Read the comment docs.