[Feature] Add roipoint_pool3d op from mmdet3d

[DCNSW]

Motivation

Add roipoint_pool3d cuda operation from mmdet3d (branch: v1.0.0.dev0).

Modification

Several files in mmcv/ops folder.

BC-breaking (Optional)

No.

Use cases (Optional)

from mmcv.ops import RoIPointPool3d

[codecov]

Codecov Report

Merging #1358 (867e073) into master (8cac7c2) will decrease coverage by 0.05%.
The diff coverage is 52.00%.

❗ Current head 867e073 differs from pull request most recent head 81bf861. Consider uploading reports for the commit 81bf861 to get more accurate results

@@            Coverage Diff             @@
##           master    #1358      +/-   ##
==========================================
- Coverage   68.59%   68.54%   -0.06%     
==========================================
  Files         164      165       +1     
  Lines       10891    10922      +31     
  Branches     1991     1994       +3     
==========================================
+ Hits         7471     7486      +15     
- Misses       3030     3043      +13     
- Partials      390      393       +3     

Flag	Coverage Δ
unittests	68.54% <52.00%> (-0.06%)	⬇️

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

Impacted Files	Coverage Δ
mmcv/ops/roipoint_pool3d.py	50.00% <50.00%> (ø)
mmcv/ops/__init__.py	100.00% <100.00%> (ø)
mmcv/runner/dist_utils.py	50.00% <0.00%> (-1.07%)	⬇️

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[grimoire]

LGTM