Add format_only option for nuScenes and Waymo evaluation

[Xiangxu-0103]

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Motivation

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Modification

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BC-breaking (Optional)

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Use cases (Optional)

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Checklist

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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

Base: 45.29% // Head: 45.29% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (c09c009) compared to base (4e28378).
Patch coverage: 28.78% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2151      +/-   ##
===========================================
- Coverage    45.29%   45.29%   -0.01%     
===========================================
  Files          255      255              
  Lines        20822    20821       -1     
  Branches      3288     3292       +4     
===========================================
- Hits          9431     9430       -1     
  Misses       10771    10771              
  Partials       620      620              

Flag	Coverage Δ
unittests	45.29% <28.78%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
mmdet3d/evaluation/metrics/nuscenes_metric.py	11.43% <20.00%> (-0.53%)	⬇️
mmdet3d/evaluation/metrics/waymo_metric.py	8.11% <31.25%> (-0.04%)	⬇️
mmdet3d/evaluation/metrics/kitti_metric.py	9.09% <40.00%> (+0.24%)	⬆️

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Need to resolve minor comments