[Feature] Spvcnn backbone

[sunjiahao1999]

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Motivation

Add spvcnn backbone.

Modification

Please briefly describe what modification is made in this PR.

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

Patch coverage: 16.16% and project coverage change: -0.14 ⚠️

Comparison is base (ee6cc04) 47.16% compared to head (a8e4157) 47.03%.

❗ Current head a8e4157 differs from pull request most recent head 445f58a. Consider uploading reports for the commit 445f58a to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2320      +/-   ##
===========================================
- Coverage    47.16%   47.03%   -0.14%     
===========================================
  Files          273      274       +1     
  Lines        22665    22764      +99     
  Branches      3535     3544       +9     
===========================================
+ Hits         10690    10706      +16     
- Misses       11294    11376      +82     
- Partials       681      682       +1     

Flag	Coverage Δ
unittests	47.03% <16.16%> (-0.14%)	⬇️

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

Impacted Files	Coverage Δ
mmdet3d/models/backbones/spvcnn_backone.py	15.30% <15.30%> (ø)
mmdet3d/models/backbones/__init__.py	100.00% <100.00%> (ø)

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