fix: Recognition dataset merging

[charlesmindee]

This PR allows recognition datasets to be merged.
Any feedback is welcome!

[codecov]

Codecov Report

Merging #376 (51a3f4a) into main (b97363c) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #376      +/-   ##
==========================================
- Coverage   96.21%   96.19%   -0.03%     
==========================================
  Files          83       83              
  Lines        3518     3524       +6     
==========================================
+ Hits         3385     3390       +5     
- Misses        133      134       +1     

Flag	Coverage Δ
unittests	96.19% <100.00%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
doctr/datasets/recognition.py	92.00% <100.00%> (+2.52%)	⬆️
...dels/detection/differentiable_binarization/base.py	91.30% <0.00%> (-0.63%)	⬇️

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[fg-mindee]

Thanks for the PR! 2 comments where I'd like to have your opinion

[fg-mindee]

Thanks!