[FIx] Set default backend_args values to None

[xiexinch]

Motivation

In MMEngine >= 0.2.0, it might directly determine what the backend is by using the data_root path.

Modification

All default backend_args values.

[codecov]

Codecov Report

Base: 83.37% // Head: 83.41% // Increases project coverage by +0.04% 🎉

Coverage data is based on head (cc33d2b) compared to base (49b062e).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head cc33d2b differs from pull request most recent head 371940f. Consider uploading reports for the commit 371940f to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2597      +/-   ##
===========================================
+ Coverage    83.37%   83.41%   +0.04%     
===========================================
  Files          146      146              
  Lines         8515     8515              
  Branches      1274     1274              
===========================================
+ Hits          7099     7103       +4     
+ Misses        1202     1198       -4     
  Partials       214      214              

Flag	Coverage Δ
unittests	83.41% <100.00%> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
mmseg/datasets/basesegdataset.py	100.00% <100.00%> (ø)
mmseg/datasets/transforms/loading.py	85.15% <100.00%> (+3.90%)	⬆️
mmseg/engine/hooks/visualization_hook.py	97.05% <100.00%> (ø)
mmseg/datasets/transforms/transforms.py	90.40% <0.00%> (-0.13%)	⬇️

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