[Fix] Fix bug when loading class name form file in custom dataset

[ShoupingShan]

expection occurs when classes variable is a string file path

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Motivation

fix a bug in custom.py, Exception occurs when classes is a string type file path .

Modification

modify classes to class_names in https://github.com/ShoupingShan/mmsegmentation/blob/396070e8d2ab96d21a798907f0139fc99ba66e5c/mmseg/datasets/custom.py#L322 and https://github.com/ShoupingShan/mmsegmentation/blob/396070e8d2ab96d21a798907f0139fc99ba66e5c/mmseg/datasets/custom.py#L333

add unit test in test.dataset.py https://github.com/ShoupingShan/mmsegmentation/blob/396070e8d2ab96d21a798907f0139fc99ba66e5c/tests/test_data/test_dataset.py#L29

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

No

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

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.

[Junjun2016]

Hi @ShoupingShan
Please fix the CI error.

[codecov]

Codecov Report

Merging #923 (d26c078) into master (85227b4) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #923      +/-   ##
==========================================
+ Coverage   89.67%   89.90%   +0.22%     
==========================================
  Files         114      117       +3     
  Lines        6431     6555     +124     
  Branches     1007     1019      +12     
==========================================
+ Hits         5767     5893     +126     
+ Misses        462      461       -1     
+ Partials      202      201       -1     

Flag	Coverage Δ
unittests	89.90% <100.00%> (+0.22%)	⬆️

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

Impacted Files	Coverage Δ
mmseg/datasets/custom.py	94.59% <100.00%> (+2.16%)	⬆️
mmseg/models/backbones/swin.py	83.73% <0.00%> (-0.30%)	⬇️
mmseg/models/necks/__init__.py	100.00% <0.00%> (ø)
mmseg/models/backbones/__init__.py	100.00% <0.00%> (ø)
mmseg/models/necks/jpu.py	100.00% <0.00%> (ø)
mmseg/models/backbones/icnet.py	97.50% <0.00%> (ø)
mmseg/models/necks/ic_neck.py	100.00% <0.00%> (ø)

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[Junjun2016]

LGTM
Thanks for your contribution again.
Looking forward to your new PR.

[xvjiarui]

We may move the import to the top of the file.

[xvjiarui]

LGTM except for one comment.
Thanks for your contribution.

[Junjun2016]

Hi @ShoupingShan
Could you please resolve the comment?

[Junjun2016]

Hi @ShoupingShan
Could you please fix the lint error?

[ShoupingShan]

Sorry about that, I have fixed the isort lint error.

[Junjun2016]

Hi @ShoupingShan
Thanks for your contribution again.
Looking forward to your new PR.