[datasets] Fix recognition parts of SynthText and IMGUR5K

[felixdittrich92]

This PR:

• fixes the mem issue for SynthText and IMGUR5K Recognition
• now we write a new dataset for both datasets to disk (fastest way)

related to:
#935 Fix for Recognition: SynthText and IMGUR5K (not beauty but it works well - no mem issue and whole speed up)

[codecov]

Codecov Report

Merging #1038 (3c1a7f7) into main (ea19161) will decrease coverage by 0.01%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##             main    #1038      +/-   ##
==========================================
- Coverage   94.92%   94.91%   -0.02%     
==========================================
  Files         135      135              
  Lines        5599     5625      +26     
==========================================
+ Hits         5315     5339      +24     
- Misses        284      286       +2     

Flag	Coverage Δ
unittests	94.91% <93.75%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
doctr/datasets/imgur5k.py	92.53% <91.66%> (-0.80%)	⬇️
doctr/datasets/synthtext.py	95.08% <95.83%> (+0.34%)	⬆️
doctr/transforms/modules/base.py	94.59% <0.00%> (ø)

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[charlesmindee]

Thanks, LGTM