[Enhance] Support with_velocity=False in CenterPoint and NuScenesDataset

[YuanxianH]

Motivation

Fix some bugs of CenterPoint and NuScenesDataset when the velocity head in the CenterHead's common_heads is ignored.

Modification

• Simply add some conditional statements to support CenterPoint without the vel common head.
• Add a config file to test the no-velocity version of CenterPoint

[codecov]

Codecov Report

Merging #1469 (f027e50) into dev (4422eaa) will decrease coverage by 0.24%.
The diff coverage is 34.48%.

@@            Coverage Diff             @@
##              dev    #1469      +/-   ##
==========================================
- Coverage   50.39%   50.15%   -0.25%     
==========================================
  Files         221      223       +2     
  Lines       18927    19124     +197     
  Branches     3118     3141      +23     
==========================================
+ Hits         9538     9591      +53     
- Misses       8811     8953     +142     
- Partials      578      580       +2     

Flag	Coverage Δ
unittests	50.15% <34.48%> (-0.25%)	⬇️

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

Impacted Files	Coverage Δ
mmdet3d/models/dense_heads/centerpoint_head.py	44.05% <9.09%> (-0.69%)	⬇️
mmdet3d/datasets/nuscenes_dataset.py	40.35% <20.00%> (-0.36%)	⬇️
mmdet3d/datasets/pipelines/dbsampler.py	74.07% <20.00%> (-1.73%)	⬇️
mmdet3d/models/detectors/centerpoint.py	17.44% <66.66%> (+1.77%)	⬆️
mmdet3d/datasets/pipelines/loading.py	90.17% <100.00%> (+0.10%)	⬆️
mmdet3d/models/middle_encoders/sparse_encoder.py	38.99% <0.00%> (-35.20%)	⬇️
mmdet3d/datasets/builder.py	42.42% <0.00%> (-0.91%)	⬇️
mmdet3d/core/post_processing/box3d_nms.py	27.89% <0.00%> (-0.88%)	⬇️
...3d/models/roi_heads/bbox_heads/parta2_bbox_head.py	67.78% <0.00%> (-0.44%)	⬇️
mmdet3d/datasets/custom_3d_seg.py	67.29% <0.00%> (-0.43%)	⬇️
... and 9 more

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