[Enhance] Provide offline annotations and polish docs

[JingweiZhang12]

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Motivation

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Modification

Please briefly describe what modification is made in this PR.

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)

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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

Patch coverage: 30.95% and project coverage change: +0.02 🎉

Comparison is base (aa10365) 46.99% compared to head (f4f308e) 47.01%.

❗ Current head f4f308e differs from pull request most recent head 02caecd. Consider uploading reports for the commit 02caecd to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2457      +/-   ##
===========================================
+ Coverage    46.99%   47.01%   +0.02%     
===========================================
  Files          275      276       +1     
  Lines        22904    22919      +15     
  Branches      3566     3569       +3     
===========================================
+ Hits         10763    10775      +12     
- Misses       11455    11462       +7     
+ Partials       686      682       -4     

Flag	Coverage Δ
unittests	47.01% <30.95%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
mmdet3d/models/segmentors/cylinder3d.py	46.66% <0.00%> (ø)
mmdet3d/models/segmentors/encoder_decoder.py	22.22% <0.00%> (-0.36%)	⬇️
mmdet3d/models/segmentors/base.py	60.78% <20.00%> (-0.76%)	⬇️
mmdet3d/models/segmentors/minkunet.py	39.39% <20.00%> (-1.24%)	⬇️
mmdet3d/models/segmentors/seg3d_tta.py	40.00% <40.00%> (ø)
mmdet3d/datasets/transforms/__init__.py	100.00% <100.00%> (ø)
mmdet3d/models/segmentors/__init__.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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