[Fix] Fix docs build error

[Xiangxu-0103]

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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)

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

Patch coverage is 19.58% of modified lines.

❗ Current head d281d4e differs from pull request most recent head 96ff2b3. Consider uploading reports for the commit 96ff2b3 to get more accurate results

Files Changed	Coverage
mmdet3d/datasets/transforms/formating.py	ø
mmdet3d/models/necks/second_fpn.py	ø
mmdet3d/visualization/local_visualizer.py	8.33%
mmdet3d/datasets/transforms/loading.py	50.00%
mmdet3d/engine/hooks/visualization_hook.py	71.42%
mmdet3d/__init__.py	100.00%
mmdet3d/datasets/transforms/transforms_3d.py	100.00%
mmdet3d/version.py	100.00%

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