[Feature] Add interpolate ops from mmdet3d

[DCNSW]

Motivation

Add interpolate 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 three_nn, three_interpolate

[codecov]

Codecov Report

Merging #1355 (ca72490) into master (8cac7c2) will decrease coverage by 0.10%.
The diff coverage is 48.97%.

❗ Current head ca72490 differs from pull request most recent head 728cb7d. Consider uploading reports for the commit 728cb7d to get more accurate results

@@            Coverage Diff             @@
##           master    #1355      +/-   ##
==========================================
- Coverage   68.59%   68.49%   -0.11%     
==========================================
  Files         164      166       +2     
  Lines       10891    10946      +55     
  Branches     1991     1993       +2     
==========================================
+ Hits         7471     7497      +26     
- Misses       3030     3057      +27     
- Partials      390      392       +2     

Flag	Coverage Δ
unittests	68.49% <48.97%> (-0.11%)	⬇️

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

Impacted Files	Coverage Δ
mmcv/ops/three_interpolate.py	42.30% <42.30%> (ø)
mmcv/ops/three_nn.py	52.38% <52.38%> (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