feat: Added character generator dataset

[fg-mindee]

Following up on #262, this PR introduces the following modifications:

• extends tensor conversion functions
• added a character generation dataset (upcoming potential update with feat: add MNIST-like characters dataset generator #408 @charlesmindee)
• updates unittests accordingly
• updated documentation

Any feedback is welcome!

[codecov]

Codecov Report

Merging #412 (5d8942f) into main (86daa0f) will decrease coverage by 0.01%.
The diff coverage is 95.60%.

@@            Coverage Diff             @@
##             main     #412      +/-   ##
==========================================
- Coverage   95.80%   95.78%   -0.02%     
==========================================
  Files          91       95       +4     
  Lines        3838     3917      +79     
==========================================
+ Hits         3677     3752      +75     
- Misses        161      165       +4     

Flag	Coverage Δ
unittests	95.78% <95.60%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
doctr/datasets/classification/base.py	90.62% <90.62%> (ø)
doctr/io/image/tensorflow.py	91.66% <94.11%> (+1.66%)	⬆️
doctr/datasets/__init__.py	100.00% <100.00%> (ø)
doctr/datasets/classification/__init__.py	100.00% <100.00%> (ø)
doctr/datasets/classification/pytorch.py	100.00% <100.00%> (ø)
doctr/datasets/classification/tensorflow.py	100.00% <100.00%> (ø)
doctr/datasets/loader.py	100.00% <100.00%> (ø)
doctr/io/image/pytorch.py	93.75% <100.00%> (+1.44%)	⬆️
... and 2 more

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