[Feature] Support lmdb format in Dataset Preparer

[gaotongxiao]

Modified from #1673

[codecov]

Codecov Report

Patch coverage: 24.70% and project coverage change: -0.50 ⚠️

Comparison is base (f250ea2) 90.06% compared to head (13c80b3) 89.56%.

❗ Current head 13c80b3 differs from pull request most recent head bcb66be. Consider uploading reports for the commit bcb66be to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #1762      +/-   ##
===========================================
- Coverage    90.06%   89.56%   -0.50%     
===========================================
  Files          190      191       +1     
  Lines        11123    11206      +83     
  Branches      1565     1576      +11     
===========================================
+ Hits         10018    10037      +19     
- Misses         798      861      +63     
- Partials       307      308       +1     

Flag	Coverage Δ
unittests	89.56% <24.70%> (-0.50%)	⬇️

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Impacted Files	Coverage Δ
...atasets/preparers/obtainers/naive_data_obtainer.py	16.82% <ø> (ø)
mmocr/datasets/preparers/dumpers/lmdb_dumper.py	21.33% <21.33%> (ø)
mmocr/datasets/preparers/config_generators/base.py	75.00% <37.50%> (-7.50%)	⬇️
...rs/config_generators/textrecog_config_generator.py	100.00% <100.00%> (ø)

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