[Features] Add stack ball query and stack group points ops

[VVsssssk]

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Motivation

Add stack ball query ops and stack group points to support Stack SA module in mmdet3d.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
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

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[grimoire]

LGTM

[grimoire]

Fix lint.

[codecov]

Codecov Report

Base: 66.78% // Head: 66.60% // Decreases project coverage by -0.17% ⚠️

Coverage data is based on head (59ab323) compared to base (2b97c52).
Patch coverage: 14.28% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.x    #2292      +/-   ##
==========================================
- Coverage   66.78%   66.60%   -0.18%     
==========================================
  Files         120      120              
  Lines        7779     7800      +21     
  Branches     1064     1067       +3     
==========================================
  Hits         5195     5195              
- Misses       2417     2438      +21     
  Partials      167      167              

Flag	Coverage Δ
unittests	66.60% <14.28%> (-0.18%)	⬇️

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

Impacted Files	Coverage Δ
mmcv/ops/group_points.py	18.18% <10.00%> (-3.10%)	⬇️
mmcv/ops/ball_query.py	40.74% <25.00%> (-9.26%)	⬇️

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