[Refactor] Refactor SPVCNN & MinkUNet

[Xiangxu-0103]

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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

Modification

This PR is ready after #2711 is merged.

• Refactor SPVCNN and MinkUNet
• Combine Cylinder3D and MinkUNet with VoxelSegmentor, so that all voxel-based segmentors will share the same structure.

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.

[codecov]

Codecov Report

Attention: 117 lines in your changes are missing coverage. Please review.

Comparison is base (5c0613b) 46.70% compared to head (1edbe4b) 46.65%.

Files	Patch %	Lines
mmdet3d/models/decode_heads/minkunet_head.py	18.42%	31 Missing ⚠️
mmdet3d/models/decode_heads/cylinder3d_head.py	32.50%	26 Missing and 1 partial ⚠️
mmdet3d/models/segmentors/voxel_segmentor.py	38.63%	26 Missing and 1 partial ⚠️
mmdet3d/models/voxel_encoders/voxel_encoder.py	33.33%	8 Missing ⚠️
mmdet3d/models/backbones/cylinder3d.py	12.50%	7 Missing ⚠️
...t3d/models/data_preprocessors/data_preprocessor.py	12.50%	7 Missing ⚠️
mmdet3d/models/backbones/minkunet_backbone.py	28.57%	5 Missing ⚠️
mmdet3d/models/data_preprocessors/voxelize.py	28.57%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2830      +/-   ##
===========================================
- Coverage    46.70%   46.65%   -0.06%     
===========================================
  Files          277      276       -1     
  Lines        23635    23658      +23     
  Branches      3704     3703       -1     
===========================================
- Hits         11039    11037       -2     
- Misses       11892    11915      +23     
- Partials       704      706       +2     

Flag	Coverage Δ
unittests	46.65% <29.94%> (-0.06%)	⬇️

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[sunjiahao1999]

Need to discuss the risk of deleting existing MinkUNet and Cylinder3D implementations.