[Feature] Add fps op from mmdet3d

[DCNSW]

Motivation

Add furthest point sample 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 furthest_point_sample, furthest_point_sample_with_dist

[codecov]

Codecov Report

Merging #1337 (a6a7156) into master (a90b397) will decrease coverage by 0.32%.
The diff coverage is 33.63%.

❗ Current head a6a7156 differs from pull request most recent head 806f086. Consider uploading reports for the commit 806f086 to get more accurate results

@@            Coverage Diff             @@
##           master    #1337      +/-   ##
==========================================
- Coverage   68.57%   68.25%   -0.33%     
==========================================
  Files         164      166       +2     
  Lines       10897    11000     +103     
  Branches     1993     1997       +4     
==========================================
+ Hits         7473     7508      +35     
- Misses       3032     3102      +70     
+ Partials      392      390       -2     

Flag	Coverage Δ
unittests	68.25% <33.63%> (-0.33%)	⬇️

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

Impacted Files	Coverage Δ
mmcv/runner/dist_utils.py	51.06% <0.00%> (+1.06%)	⬆️
mmcv/ops/points_sampler.py	25.33% <25.33%> (ø)
mmcv/ops/furthest_point_sample.py	50.00% <50.00%> (ø)
mmcv/ops/__init__.py	100.00% <100.00%> (ø)

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[zhouzaida]

LGTM

[grimoire]

LGTM

[ZwwWayne]

Suggest migrating all DIVUP to GET_BLOCKS as they essentially do the same thing but GET_BLOCKS looks safer.