[Enhance] Support different colors for different classes in visualization

[JingweiZhang12]

This PR waits for the merging of showing multi-view image

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.

[codecov]

Codecov Report

Patch coverage: 70.00% and project coverage change: -0.02 ⚠️

Comparison is base (35fd839) 47.01% compared to head (cf02cdc) 46.99%.

❗ Current head cf02cdc differs from pull request most recent head 920a897. Consider uploading reports for the commit 920a897 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2500      +/-   ##
===========================================
- Coverage    47.01%   46.99%   -0.02%     
===========================================
  Files          276      276              
  Lines        23105    23144      +39     
  Branches      3602     3611       +9     
===========================================
+ Hits         10862    10877      +15     
- Misses       11557    11579      +22     
- Partials       686      688       +2     

Flag	Coverage Δ
unittests	46.99% <70.00%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
mmdet3d/datasets/kitti_dataset.py	55.10% <ø> (ø)
mmdet3d/datasets/lyft_dataset.py	78.26% <ø> (ø)
mmdet3d/datasets/nuscenes_dataset.py	47.61% <ø> (ø)
mmdet3d/datasets/s3dis_dataset.py	78.08% <ø> (ø)
mmdet3d/datasets/scannet_dataset.py	82.75% <ø> (ø)
mmdet3d/datasets/sunrgbd_dataset.py	69.44% <ø> (ø)
...et3d/models/dense_heads/anchor_free_mono3d_head.py	81.81% <ø> (-0.10%)	⬇️
mmdet3d/models/dense_heads/centerpoint_head.py	21.53% <ø> (-0.25%)	⬇️
mmdet3d/models/dense_heads/groupfree3d_head.py	21.57% <ø> (-0.19%)	⬇️
mmdet3d/models/dense_heads/vote_head.py	18.27% <ø> (-0.28%)	⬇️
... and 25 more

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