[Enhance] Format semantickitti label mapping

[sunjiahao1999]

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Motivation

Format semantickitti label mapping and fix some loading bug.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

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If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

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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.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (cf6f473) 46.86% compared to head (793bebc) 46.88%.

❗ Current head 793bebc differs from pull request most recent head 364d758. Consider uploading reports for the commit 364d758 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2323      +/-   ##
===========================================
+ Coverage    46.86%   46.88%   +0.01%     
===========================================
  Files          264      264              
  Lines        22173    22175       +2     
  Branches      3482     3482              
===========================================
+ Hits         10392    10396       +4     
+ Misses       11118    11116       -2     
  Partials       663      663              

Flag	Coverage Δ
unittests	46.88% <100.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
mmdet3d/datasets/__init__.py	100.00% <100.00%> (ø)
mmdet3d/datasets/seg3d_dataset.py	72.91% <100.00%> (+4.83%)	⬆️
mmdet3d/datasets/semantickitti_dataset.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[Xiangxu-0103]

Should we rename this file to semantickitti-seg.py?

[ZwwWayne]

we can do that

[sunjiahao1999]

I think semantickitti already indicates that is the semantic version of the kitti dataset.

[sunjiahao1999]

but we should repalce SemanticKITTIDataset with SemanticKittiDataset

[ZwwWayne]

we can do that