[Enhancement] Support format_only for LyftDataset.

[Xiangxu-0103]

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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

Modification

• Support format_only for LyftDataset
• Update docstring.

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: 16.00% and project coverage change: -0.02 ⚠️

Comparison is base (cf6f473) 46.86% compared to head (39f8be1) 46.85%.

❗ Current head 39f8be1 differs from pull request most recent head ff6f05f. Consider uploading reports for the commit ff6f05f to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2333      +/-   ##
===========================================
- Coverage    46.86%   46.85%   -0.02%     
===========================================
  Files          264      264              
  Lines        22173    22177       +4     
  Branches      3482     3484       +2     
===========================================
- Hits         10392    10391       -1     
- Misses       11118    11123       +5     
  Partials       663      663              

Flag	Coverage Δ
unittests	46.85% <16.00%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
mmdet3d/evaluation/metrics/instance_seg_metric.py	94.44% <ø> (ø)
mmdet3d/evaluation/metrics/kitti_metric.py	9.09% <0.00%> (ø)
mmdet3d/evaluation/metrics/nuscenes_metric.py	11.43% <0.00%> (ø)
mmdet3d/evaluation/metrics/waymo_metric.py	8.11% <0.00%> (ø)
mmdet3d/evaluation/metrics/lyft_metric.py	16.36% <12.50%> (-1.14%)	⬇️
mmdet3d/evaluation/metrics/indoor_metric.py	60.65% <50.00%> (+0.97%)	⬆️

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