[Feature] Add typehint to mmdet3d/models/backbones.

[Lum1104]

Motivation

Add typehint to the code and make it more readable.

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: 65.97% and project coverage change: +0.03 🎉

Comparison is base (b4b9af6) 47.14% compared to head (4194e42) 47.17%.

❗ Current head 4194e42 differs from pull request most recent head ae2975e. Consider uploading reports for the commit ae2975e to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2464      +/-   ##
===========================================
+ Coverage    47.14%   47.17%   +0.03%     
===========================================
  Files          276      276              
  Lines        22979    23032      +53     
  Branches      3569     3582      +13     
===========================================
+ Hits         10833    10866      +33     
- Misses       11464    11484      +20     
  Partials       682      682              

Flag	Coverage Δ
unittests	47.17% <65.97%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
mmdet3d/apis/inference.py	14.45% <0.00%> (-0.18%)	⬇️
mmdet3d/datasets/transforms/loading.py	44.91% <ø> (ø)
mmdet3d/datasets/transforms/transforms_3d.py	40.09% <0.00%> (-0.13%)	⬇️
mmdet3d/models/decode_heads/cylinder3d_head.py	51.35% <ø> (ø)
mmdet3d/models/decode_heads/minkunet_head.py	50.00% <0.00%> (ø)
mmdet3d/structures/bbox_3d/utils.py	78.57% <ø> (ø)
mmdet3d/utils/setup_env.py	81.81% <ø> (ø)
mmdet3d/visualization/local_visualizer.py	33.44% <0.00%> (-0.23%)	⬇️
...t3d/models/data_preprocessors/data_preprocessor.py	31.46% <9.09%> (+0.16%)	⬆️
mmdet3d/engine/hooks/visualization_hook.py	58.02% <37.50%> (-11.84%)	⬇️
... and 10 more

... and 1 file with indirect coverage changes

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[Xiangxu-0103]

Please use more standardized and appropriate typehints, you can take other modules as references.