add primitive head

[encore-zhou]

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[codecov]

Codecov Report

Merging #53 into master will increase coverage by 1.85%.
The diff coverage is 23.75%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   48.47%   50.33%   +1.85%     
==========================================
  Files         139      141       +2     
  Lines        8435     8780     +345     
  Branches     1304     1355      +51     
==========================================
+ Hits         4089     4419     +330     
+ Misses       4087     4081       -6     
- Partials      259      280      +21     

Flag	Coverage Δ
#unittests	50.33% <23.75%> (+1.85%)	⬆️

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

Impacted Files	Coverage Δ
...3d/core/bbox/samplers/iou_neg_piecewise_sampler.py	22.22% <0.00%> (ø)
mmdet3d/models/dense_heads/free_anchor3d_head.py	19.76% <0.00%> (ø)
mmdet3d/models/dense_heads/vote_head.py	22.42% <0.00%> (ø)
mmdet3d/models/model_utils/vote_module.py	78.26% <0.00%> (ø)
mmdet3d/ops/iou3d/iou3d_utils.py	27.77% <0.00%> (ø)
...et3d/models/roi_heads/mask_heads/primitive_head.py	10.50% <10.50%> (ø)
mmdet3d/models/backbones/multi_backbone.py	22.00% <22.00%> (ø)
mmdet3d/models/middle_encoders/sparse_encoder.py	67.85% <55.55%> (-4.15%)	⬇️
mmdet3d/models/voxel_encoders/pillar_encoder.py	56.89% <55.55%> (-1.29%)	⬇️
mmdet3d/datasets/pipelines/loading.py	71.55% <93.54%> (+1.99%)	⬆️
... and 16 more

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