[Docs] Add docs and README for Cylinder3D

[xizaoqu]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Add docs and README for Cylinder3D.

Modification

Add docs and README for Cylinder3D.

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.

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.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Patch coverage: 61.54% and project coverage change: +0.90 🎉

Comparison is base (d84e081) 46.37% compared to head (8475203) 47.27%.

❗ Current head 8475203 differs from pull request most recent head f9a3a5e. Consider uploading reports for the commit f9a3a5e to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2350      +/-   ##
===========================================
+ Coverage    46.37%   47.27%   +0.90%     
===========================================
  Files          259      270      +11     
  Lines        21390    22513    +1123     
  Branches      3379     3513     +134     
===========================================
+ Hits          9919    10643     +724     
- Misses       10827    11193     +366     
- Partials       644      677      +33     

Flag	Coverage Δ
unittests	47.27% <61.54%> (+0.90%)	⬆️

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

Impacted Files	Coverage Δ
mmdet3d/datasets/s3dis_dataset.py	78.08% <ø> (ø)
mmdet3d/datasets/scannet_dataset.py	82.75% <ø> (ø)
mmdet3d/datasets/transforms/__init__.py	100.00% <ø> (ø)
mmdet3d/datasets/waymo_dataset.py	17.72% <ø> (ø)
mmdet3d/evaluation/__init__.py	100.00% <ø> (ø)
...tion/functional/waymo_utils/prediction_to_waymo.py	0.00% <0.00%> (ø)
mmdet3d/evaluation/metrics/instance_seg_metric.py	94.44% <ø> (ø)
mmdet3d/evaluation/metrics/seg_metric.py	62.50% <0.00%> (ø)
mmdet3d/evaluation/metrics/waymo_metric.py	8.11% <0.00%> (ø)
mmdet3d/models/dense_heads/centerpoint_head.py	21.77% <0.00%> (ø)
... and 66 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[Tai-Wang]

Just need to fix minor comments

[JingweiZhang12]

Please fix the conflicts

[JingweiZhang12]

LGTM