[feature] Part 2 from use datasets for recognition

[felixdittrich92]

This PR:

• enables all datasets to be used for recognition task (without obj_detection one)

• add corresponding tests

• one special case: SynthText (to big will lead to memory overflow) -> saved in cache for faster reload and avoiding memory explosions 😅

• all other works fine without caching

Issues:
Closes #867
(Documentation for this will follow in another PR #855 )

Any feedback is welcome 🤗

[codecov]

Codecov Report

Merging #891 (ab2b437) into main (1a4e4df) will increase coverage by 0.00%.
The diff coverage is 98.48%.

@@           Coverage Diff           @@
##             main     #891   +/-   ##
=======================================
  Coverage   94.71%   94.72%           
=======================================
  Files         135      135           
  Lines        5375     5440   +65     
=======================================
+ Hits         5091     5153   +62     
- Misses        284      287    +3     

Flag	Coverage Δ
unittests	94.72% <98.48%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
doctr/datasets/synthtext.py	94.73% <92.59%> (-2.41%)	⬇️
doctr/datasets/cord.py	97.72% <100.00%> (+0.29%)	⬆️
doctr/datasets/funsd.py	97.36% <100.00%> (+0.39%)	⬆️
doctr/datasets/ic03.py	97.72% <100.00%> (+0.35%)	⬆️
doctr/datasets/ic13.py	96.77% <100.00%> (+0.62%)	⬆️
doctr/datasets/iiit5k.py	96.96% <100.00%> (+0.19%)	⬆️
doctr/datasets/imgur5k.py	93.33% <100.00%> (+0.83%)	⬆️
doctr/datasets/sroie.py	97.29% <100.00%> (+0.42%)	⬆️
doctr/datasets/svhn.py	95.23% <100.00%> (+0.50%)	⬆️
doctr/datasets/svt.py	97.61% <100.00%> (+0.39%)	⬆️
... and 2 more

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[felixdittrich92]

@charlesmindee wdyt?

[charlesmindee]

Thanks!