[Dataset Preparer] MJSynth

[gaotongxiao]

This PR also modifies the way ICDARTxtTextRecogAnnParser generates img_path, which would not affect the existing implementations through testing.

[codecov]

Codecov Report

Patch coverage: 75.00% and no project coverage change.

Comparison is base (d80df99) 89.35% compared to head (fb70345) 89.36%.

❗ Current head fb70345 differs from pull request most recent head 985d07b. Consider uploading reports for the commit 985d07b to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           dev-1.x    #1791   +/-   ##
========================================
  Coverage    89.35%   89.36%           
========================================
  Files          192      192           
  Lines        11278    11279    +1     
  Branches      1596     1596           
========================================
+ Hits         10078    10079    +1     
  Misses         886      886           
  Partials       314      314           

Flag	Coverage Δ
unittests	89.36% <75.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...atasets/preparers/obtainers/naive_data_obtainer.py	16.21% <0.00%> (ø)
...ocr/datasets/preparers/packers/textrecog_packer.py	81.53% <100.00%> (+0.28%)	⬆️
...ocr/datasets/preparers/parsers/icdar_txt_parser.py	79.24% <100.00%> (ø)

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