[Feature] Support persistent_workers in DataLoader (PyTorch>=1.7.0)

[xvjiarui]

No description provided.

[codecov]

Codecov Report

Merging #646 (cf4aa03) into master (98067be) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #646      +/-   ##
==========================================
+ Coverage   85.77%   85.82%   +0.04%     
==========================================
  Files         103      103              
  Lines        5307     5304       -3     
  Branches      857      856       -1     
==========================================
  Hits         4552     4552              
+ Misses        583      581       -2     
+ Partials      172      171       -1     

Flag	Coverage Δ
unittests	85.80% <80.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
mmseg/datasets/builder.py	89.33% <100.00%> (+3.43%)	⬆️

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