[Feature] Support multi-modality visualization (demos and dataset show function)

[Wuziyi616]

Support visualizing 3D bounding boxes on the point cloud, while projecting the boxes onto 2D plane and drawing on the image. Both outdoor (lidar) and indoor (depth) point clouds are supported. The usage is similar to pcd_demo.py.

[Wuziyi616]

Currently the user should input pre-computed camera matrix (pre-processed info file).

[Wuziyi616]

Some demo examples (MVX-Net on KITTI and ImVoteNet on SUN RGB-D dataset).

[codecov]

Codecov Report

Merging #405 (9551792) into master (825f47a) will increase coverage by 0.27%.
The diff coverage is 62.80%.

❗ Current head 9551792 differs from pull request most recent head 9922c9d. Consider uploading reports for the commit 9922c9d to get more accurate results

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
+ Coverage   50.17%   50.45%   +0.27%     
==========================================
  Files         184      184              
  Lines       13177    13320     +143     
  Branches     2097     2127      +30     
==========================================
+ Hits         6611     6720     +109     
- Misses       6130     6158      +28     
- Partials      436      442       +6     

Flag	Coverage Δ
unittests	50.45% <62.80%> (+0.27%)	⬆️

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

Impacted Files	Coverage Δ
mmdet3d/datasets/lyft_dataset.py	65.26% <0.00%> (ø)
mmdet3d/datasets/nuscenes_dataset.py	33.48% <0.00%> (ø)
mmdet3d/apis/inference.py	56.00% <34.84%> (-23.69%)	⬇️
mmdet3d/core/visualizer/show_result.py	65.17% <57.14%> (-1.86%)	⬇️
mmdet3d/datasets/sunrgbd_dataset.py	75.60% <83.33%> (+3.00%)	⬆️
mmdet3d/core/visualizer/open3d_vis.py	27.94% <89.79%> (+27.94%)	⬆️
mmdet3d/apis/__init__.py	100.00% <100.00%> (ø)
mmdet3d/core/visualizer/__init__.py	100.00% <100.00%> (ø)
mmdet3d/datasets/kitti_dataset.py	76.22% <100.00%> (+0.45%)	⬆️
mmdet3d/datasets/scannet_dataset.py	95.16% <100.00%> (ø)
... and 2 more

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[Tai-Wang]

Please also add related instructions into the documentation.

[Wuziyi616]

Please also add related instructions into the documentation.

I have add information about multi-modality demo into getting_started.md and add a README file to demo.

[Wuziyi616]

Apart from providing demos for multi-modality visualization, I also integrate it to the show function of dataset class (SUN RGB-D and KITTI dataset).

[Wuziyi616]

Resolve conflicts with master branch.