[Feature] Support PolarMix augmentation

[Xiangxu-0103]

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Motivation

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Modification

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BC-breaking (Optional)

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Checklist

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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

Base: 47.09% // Head: 46.96% // Decreases project coverage by -0.14% ⚠️

Coverage data is based on head (3e32592) compared to base (6318a11).
Patch coverage: 38.01% of modified lines in pull request are covered.

❗ Current head 3e32592 differs from pull request most recent head 6a80afd. Consider uploading reports for the commit 6a80afd to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2265      +/-   ##
===========================================
- Coverage    47.09%   46.96%   -0.14%     
===========================================
  Files          261      262       +1     
  Lines        21679    21964     +285     
  Branches      3425     3460      +35     
===========================================
+ Hits         10210    10315     +105     
- Misses       10816    10987     +171     
- Partials       653      662       +9     

Flag	Coverage Δ
unittests	46.96% <38.01%> (-0.14%)	⬇️

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

Impacted Files	Coverage Δ
mmdet3d/datasets/transforms/__init__.py	100.00% <ø> (ø)
mmdet3d/models/backbones/mink_resnet.py	25.00% <ø> (ø)
mmdet3d/models/dense_heads/fcaf3d_head.py	16.23% <ø> (ø)
mmdet3d/models/detectors/mink_single_stage.py	43.58% <ø> (ø)
mmdet3d/models/voxel_encoders/voxel_encoder.py	38.46% <6.94%> (-12.08%)	⬇️
...t3d/models/data_preprocessors/data_preprocessor.py	39.10% <17.24%> (-5.05%)	⬇️
mmdet3d/models/data_preprocessors/voxelize.py	33.92% <33.92%> (ø)
mmdet3d/datasets/seg3d_dataset.py	68.08% <66.66%> (-0.10%)	⬇️
mmdet3d/datasets/transforms/transforms_3d.py	36.80% <80.55%> (+3.47%)	⬆️
mmdet3d/models/voxel_encoders/__init__.py	100.00% <100.00%> (ø)

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