[Feature] Add voxel ops from mmdet3d

[DCNSW]

Motivation

Add voxel 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 DynamicScatter, dynamic_scatter, Voxelization, voxelization

[codecov]

Codecov Report

Merging #1381 (29598d8) into master (8cac7c2) will decrease coverage by 0.35%.
The diff coverage is 30.76%.

❗ Current head 29598d8 differs from pull request most recent head 17d89f3. Consider uploading reports for the commit 17d89f3 to get more accurate results

@@            Coverage Diff             @@
##           master    #1381      +/-   ##
==========================================
- Coverage   68.59%   68.24%   -0.36%     
==========================================
  Files         164      166       +2     
  Lines       10891    10995     +104     
  Branches     1991     1996       +5     
==========================================
+ Hits         7471     7503      +32     
- Misses       3030     3102      +72     
  Partials      390      390              

Flag	Coverage Δ
unittests	68.24% <30.76%> (-0.36%)	⬇️

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Impacted Files	Coverage Δ
mmcv/ops/voxelize.py	27.45% <27.45%> (ø)
mmcv/ops/scatter_points.py	31.37% <31.37%> (ø)
mmcv/ops/__init__.py	100.00% <100.00%> (ø)

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[grimoire]

Better add a voxelize test.

[grimoire]

Voxelization test failed with:

E           RuntimeError: points.device().is_cpu() INTERNAL ASSERT FAILED at "/home/PJLAB/yaoqian/space/mmcv/mmcv/ops/csrc/pytorch/voxelization_cpu.cpp":99, please report a bug to PyTorch. points must be a CPU tensor