[Doc] Add a tutorial for nuScenes dataset

[Tai-Wang]

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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.
Add a tutorial for nuScenes dataset.

Modification

Please briefly describe what modification is made in this PR.

1. Add a tutorial for nuScenes dataset.
2. Add a Chinese placeholder for the nuScenes tutorial.

BC-breaking (Optional)

Does the modification introduce changes that break the back-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.
No.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #793 (ad62ac0) into master (9312b34) will increase coverage by 0.09%.
The diff coverage is 65.09%.

❗ Current head ad62ac0 differs from pull request most recent head ec8d944. Consider uploading reports for the commit ec8d944 to get more accurate results

@@            Coverage Diff             @@
##           master     #793      +/-   ##
==========================================
+ Coverage   48.94%   49.03%   +0.09%     
==========================================
  Files         208      210       +2     
  Lines       15884    15964      +80     
  Branches     2538     2547       +9     
==========================================
+ Hits         7774     7828      +54     
- Misses       7608     7634      +26     
  Partials      502      502              

Flag	Coverage Δ
unittests	49.03% <65.09%> (+0.09%)	⬆️

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Impacted Files	Coverage Δ
mmdet3d/apis/test.py	15.78% <0.00%> (ø)
mmdet3d/core/visualizer/show_result.py	66.38% <ø> (ø)
mmdet3d/ops/group_points/group_points.py	31.18% <0.00%> (ø)
mmdet3d/ops/pointnet_modules/paconv_sa_module.py	28.57% <ø> (ø)
mmdet3d/models/segmentors/encoder_decoder.py	20.94% <19.04%> (-0.70%)	⬇️
mmdet3d/models/segmentors/base.py	29.78% <33.33%> (+0.89%)	⬆️
mmdet3d/models/decode_heads/paconv_head.py	43.75% <43.75%> (ø)
mmdet3d/datasets/pipelines/transforms_3d.py	90.39% <60.00%> (-0.48%)	⬇️
mmdet3d/datasets/builder.py	62.06% <100.00%> (+1.35%)	⬆️
mmdet3d/models/decode_heads/__init__.py	100.00% <100.00%> (ø)
... and 7 more

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[Wuziyi616]

Do you think it's reasonable to explain a bit about nuScenes' coordinate system, e.g. NuscBox's definition with our coordinate system? There are several related operation in the codebase. I understand that after refactoring the hack of rotation, the related code is less now. They exist in, e.g. nuscenes_converter.py and output_to_nusc_box in nusc_mono_dataset.

[Tai-Wang]

Do you think it's reasonable to explain a bit about nuScenes' coordinate system, e.g. NuscBox's definition with our coordinate system? There are several related operation in the codebase. I understand that after refactoring the hack of rotation, the related code is less now. They exist in, e.g. nuscenes_converter.py and output_to_nusc_box in nusc_mono_dataset.

Yes, I add a "Notes" section to explain questions like that.

[Wuziyi616]

Great tutorial!