[Enhance] Use Inferencer to implement Demo

[sunjiahao1999]

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Motivation

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Modification

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BC-breaking (Optional)

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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.

[Tai-Wang]

Please set the default visualization option to False, because users may have trouble installing open3d on clusters but they need test the inference demo.

By the way, update the doc if necessary.

[codecov]

Codecov Report

Attention: 260 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
mmdet3d/__init__.py	100.00% <100.00%> (ø)
mmdet3d/datasets/transforms/formating.py	58.33% <ø> (ø)
mmdet3d/datasets/transforms/transforms_3d.py	40.09% <100.00%> (+0.11%)	⬆️
mmdet3d/models/necks/second_fpn.py	90.62% <ø> (ø)
mmdet3d/version.py	61.53% <100.00%> (ø)
mmdet3d/apis/inference.py	12.97% <0.00%> (ø)
mmdet3d/engine/hooks/visualization_hook.py	50.98% <71.42%> (+1.50%)	⬆️
mmdet3d/datasets/transforms/loading.py	45.75% <42.85%> (+0.84%)	⬆️
mmdet3d/apis/inferencers/mono_det3d_inferencer.py	79.41% <70.58%> (-10.42%)	⬇️
mmdet3d/apis/inferencers/lidar_seg3d_inferencer.py	45.00% <10.52%> (-10.72%)	⬇️
... and 4 more

... and 4 files with indirect coverage changes

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