[Ops]Add three nn vector pool ops

[VVsssssk]

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Motivation

Add three nn vector pool ops in 2.x branch.

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)

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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.

[codecov]

Codecov Report

Base: 66.70% // Head: 66.61% // Decreases project coverage by -0.08% ⚠️

Coverage data is based on head (3a39c2a) compared to base (d510b8b).
Patch coverage: 40.00% of modified lines in pull request are covered.

❗ Current head 3a39c2a differs from pull request most recent head f0b8a18. Consider uploading reports for the commit f0b8a18 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##              2.x    #2494      +/-   ##
==========================================
- Coverage   66.70%   66.61%   -0.09%     
==========================================
  Files         120      121       +1     
  Lines        7817     7842      +25     
  Branches     1072     1073       +1     
==========================================
+ Hits         5214     5224      +10     
- Misses       2436     2451      +15     
  Partials      167      167              

Flag	Coverage Δ
unittests	66.61% <40.00%> (-0.09%)	⬇️

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

Impacted Files	Coverage Δ
mmcv/ops/three_nn_vector_pool.py	37.50% <37.50%> (ø)
mmcv/ops/__init__.py	100.00% <100.00%> (ø)

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[CLAassistant]

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