[Feature]Add Decathlon dataset

[MeowZheng]

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Motivation

1. Add Decathlon to load msd dataset

Modification

1. Add mmseg/datasets/basesegdataset.py and its ut

BC-breaking (Optional)

No

Use cases (Optional)

load msd dataset info

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMDet3D.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Base: 83.51% // Head: 83.57% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (9132fd9) compared to base (ac9ee8c).
Patch coverage: 93.93% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2227      +/-   ##
===========================================
+ Coverage    83.51%   83.57%   +0.06%     
===========================================
  Files          139      140       +1     
  Lines         7927     7958      +31     
  Branches      1186     1191       +5     
===========================================
+ Hits          6620     6651      +31     
+ Misses        1118     1117       -1     
- Partials       189      190       +1     

Flag	Coverage Δ
unittests	83.57% <93.93%> (+0.06%)	⬆️

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

Impacted Files	Coverage Δ
mmseg/datasets/decathlon.py	93.33% <93.33%> (ø)
mmseg/datasets/__init__.py	100.00% <100.00%> (ø)
mmseg/datasets/basesegdataset.py	100.00% <100.00%> (ø)
mmseg/models/backbones/timm_backbone.py	45.83% <0.00%> (+8.33%)	⬆️

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