refactor: ⚡ Speeded up CTC decoding in PyTorch by x10

[fg-mindee]

This PR introduces the following modifications:

• extends arg type support of decode_sequence
• refactored CTC decoding in PyTorch bringing a 10x speedup
• updated unittests

Any feedback is welcome!

[codecov]

Codecov Report

Merging #633 (bbf5676) into main (16d842e) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #633      +/-   ##
==========================================
+ Coverage   96.05%   96.07%   +0.02%     
==========================================
  Files         112      112              
  Lines        4359     4357       -2     
==========================================
- Hits         4187     4186       -1     
+ Misses        172      171       -1     

Flag	Coverage Δ
unittests	96.07% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
doctr/datasets/utils.py	94.73% <100.00%> (+2.42%)	⬆️
doctr/models/recognition/crnn/pytorch.py	100.00% <100.00%> (ø)

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[charlesmindee]

thanks for the refactor!