[Fix] Ensure that items in dataset have the same order across multi machine

[uni19]

Motivation

ensure items in dataset have the same order across multi machine

Modification

sort items according to their filenames when using scandir

[Junjun2016]

Hi @uni19
Could you please fix the lint error?

[codecov]

Codecov Report

Merging #780 (c965947) into master (58f5dbc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head c965947 differs from pull request most recent head f484482. Consider uploading reports for the commit f484482 to get more accurate results

@@           Coverage Diff           @@
##           master     #780   +/-   ##
=======================================
  Coverage   85.37%   85.37%           
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  Files         107      107           
  Lines        5847     5848    +1     
  Branches      960      961    +1     
=======================================
+ Hits         4992     4993    +1     
  Misses        673      673           
  Partials      182      182           

Flag	Coverage Δ
unittests	85.36% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
mmseg/datasets/custom.py	89.88% <100.00%> (+0.06%)	⬆️

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[Junjun2016]

LGTM

[OpenMMLab-Coodinator]

Hi! @uni19 First of all, we want to express our gratitude for your significant PR in this project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

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