[Feature] Support ISPRS Potsdam Dataset.

[Junjue-Wang]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Enhance the dataset diversity for MMSegmentation in Remote Sensing.

Modification

Please briefly describe what modification is made in this PR.

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.

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.

[MengzhangLI]

Hi, @Junjue-Wang Thanks for your nice work.

We would review this pr ASAP.

Best,

[codecov]

Codecov Report

Merging #1097 (c161c14) into master (6c3e63e) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1097      +/-   ##
==========================================
+ Coverage   90.10%   90.13%   +0.02%     
==========================================
  Files         126      127       +1     
  Lines        7430     7447      +17     
  Branches     1239     1240       +1     
==========================================
+ Hits         6695     6712      +17     
  Misses        530      530              
  Partials      205      205              

Flag	Coverage Δ
unittests	90.13% <100.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
mmseg/core/evaluation/class_names.py	90.90% <100.00%> (+2.02%)	⬆️
mmseg/datasets/__init__.py	100.00% <100.00%> (ø)
mmseg/datasets/potsdam.py	100.00% <100.00%> (ø)

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[Junjun2016]

Hi @Junjue-Wang
Thanks for your contribution again.
Please fix the lint error and refine the PR message.

[Junjue-Wang]

@MengzhangLI I have already added the vaihingen api

[MengzhangLI]

Copy that. I would review & test your code in the next few days.

Thank you.

[MeowZheng]

LGTM