[Feature] Support MinkowskiEngine with MinkResNet

[filaPro]

Motivation

Support MinkowskiEngine as a part of FCAF3D development plan. This sparse convolution backend can be used for other detection or segmentation methods.

Modification

Add docs about MinkowskiEngine installation and MinkResNet model with test.

BC-breaking (Optional)

No.

[codecov]

Codecov Report

Merging #1422 (07fbe04) into dev (023c88b) will decrease coverage by 0.27%.
The diff coverage is 27.58%.

@@            Coverage Diff             @@
##              dev    #1422      +/-   ##
==========================================
- Coverage   51.17%   50.89%   -0.28%     
==========================================
  Files         212      217       +5     
  Lines       18284    18567     +283     
  Branches     2980     3031      +51     
==========================================
+ Hits         9356     9450      +94     
- Misses       8371     8549     +178     
- Partials      557      568      +11     

Flag	Coverage Δ
unittests	50.89% <27.58%> (-0.28%)	⬇️

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

Impacted Files	Coverage Δ
mmdet3d/models/backbones/mink_resnet.py	26.31% <26.31%> (ø)
mmdet3d/models/backbones/__init__.py	100.00% <100.00%> (ø)
mmdet3d/models/detectors/two_stage.py	80.00% <0.00%> (-20.00%)	⬇️
mmdet3d/models/builder.py	84.93% <0.00%> (-3.45%)	⬇️
mmdet3d/models/roi_heads/h3d_roi_head.py	35.71% <0.00%> (-1.13%)	⬇️
...ls/roi_heads/mask_heads/pointwise_semantic_head.py	29.33% <0.00%> (-0.93%)	⬇️
mmdet3d/models/roi_heads/point_rcnn_roi_head.py	25.96% <0.00%> (-0.71%)	⬇️
...et3d/models/roi_heads/part_aggregation_roi_head.py	33.02% <0.00%> (-0.61%)	⬇️
mmdet3d/models/dense_heads/ssd_3d_head.py	14.67% <0.00%> (-0.47%)	⬇️
mmdet3d/models/segmentors/encoder_decoder.py	21.57% <0.00%> (-0.42%)	⬇️
... and 27 more

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[ZCMax]

Once one comment left fixed, the PR is ready to merge.

[filaPro]

Looks like there is no BACKBONES in mmdet3d.models. Should I import from mmdet3d.models.builder, or wait untill the related PR is merged?

[ZCMax]

Looks like there is no BACKBONES in mmdet3d.models. Should I import from mmdet3d.models.builder, or wait untill the related PR is merged?

yes, sorry it's my mistake, just import from mmdet3d.models.builder

[ZwwWayne]

Hi @filaPro ,
Thanks for the kind contribution. The overall looks good to me.
There are a few comments before merge. Further, you may consider add the package in https://github.com/open-mmlab/mmdetection3d/blob/master/requirements/optional.txt

[filaPro]

Hi @ZwwWayne,
I can replace

apt-get install -y python3-dev libopenblas-dev
pip install ninja
pip install -U git+https://github.com/NVIDIA/MinkowskiEngine --install-option="--blas=openblas" -v --no-deps

with

conda install openblas-devel -c anaconda
pip install -U git+https://github.com/NVIDIA/MinkowskiEngine -v --no-deps --install-option="--blas_include_dirs=/opt/conda/include" --install-option="--blas=openblas"

Is it ok? Also don't quite understand which of these 2 lines can we add to requirements/optional.txt?

[ZwwWayne]

I think we can simply use these two lines.
If so, we may skip adding the optional.txt since it requires extra commands and dependencies.