[Feature] Panoptic segmentation metric

[xizaoqu]

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Motivation

Add 3D panoptic segmentation metric for semanticKITTI.

Modification

Add 3D panoptic segmentation metric. NOTE: To verify the metric, this PR also includes the corresponding utils, but there are not merged into the repo yet.

BC-breaking (Optional)

Does the modification introduce changes that break the back-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.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[JingweiZhang12]

Please fix the lint

[xizaoqu]

Please fix the lint

I've fixed them. Please check.

[JingweiZhang12]

The code LGTM. Please pull the latest code which has fixed the lint about isort. @xizaoqu

[JingweiZhang12]

@sunjiahao1999 Please review this PR.

[sunjiahao1999]

Fix lint error.

[codecov]

Codecov Report

Base: 46.39% // Head: 47.04% // Increases project coverage by +0.64% 🎉

Coverage data is based on head (bb8bad3) compared to base (6810244).
Patch coverage: 90.94% of modified lines in pull request are covered.

❗ Current head bb8bad3 differs from pull request most recent head 30d40b7. Consider uploading reports for the commit 30d40b7 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2230      +/-   ##
===========================================
+ Coverage    46.39%   47.04%   +0.64%     
===========================================
  Files          259      261       +2     
  Lines        21401    21635     +234     
  Branches      3379     3417      +38     
===========================================
+ Hits          9930    10178     +248     
+ Misses       10827    10807      -20     
- Partials       644      650       +6     

Flag	Coverage Δ
unittests	47.04% <90.94%> (+0.64%)	⬆️

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

Impacted Files	Coverage Δ
mmdet3d/evaluation/__init__.py	100.00% <ø> (ø)
mmdet3d/visualization/local_visualizer.py	33.90% <0.00%> (-0.36%)	⬇️
mmdet3d/models/segmentors/encoder_decoder.py	17.20% <72.72%> (ø)
mmdet3d/models/segmentors/base.py	57.69% <75.00%> (+5.06%)	⬆️
mmdet3d/evaluation/metrics/panoptic_seg_metric.py	90.00% <90.00%> (ø)
mmdet3d/datasets/dataset_wrappers.py	92.64% <92.59%> (+67.64%)	⬆️
mmdet3d/evaluation/functional/panoptic_seg_eval.py	93.45% <93.45%> (ø)
mmdet3d/evaluation/functional/__init__.py	100.00% <100.00%> (ø)
mmdet3d/evaluation/metrics/__init__.py	100.00% <100.00%> (ø)
mmdet3d/models/decode_heads/decode_head.py	78.00% <100.00%> (ø)
... and 5 more

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[ZwwWayne]

last round, the overall LGTM

[xizaoqu]

last round, the overall LGTM

I've fixed the problems.