[Fix] NMS for Point RCNN

[filaPro]

Motivation

Fix #1409 . Now tests/test_models/test_heads/test_heads.py::test_point_rcnn_rpnhead_getboxes and tests/test_models/test_detectors.py::test_point_rcnn are passed successfully.

BC-breaking (Optional)

No

[codecov]

Codecov Report

Merging #1418 (a2ae474) into dev (023c88b) will increase coverage by 0.02%.
The diff coverage is 10.00%.

@@            Coverage Diff             @@
##              dev    #1418      +/-   ##
==========================================
+ Coverage   51.17%   51.19%   +0.02%     
==========================================
  Files         212      212              
  Lines       18284    18289       +5     
  Branches     2980     2980              
==========================================
+ Hits         9356     9363       +7     
+ Misses       8371     8369       -2     
  Partials      557      557              

Flag	Coverage Δ
unittests	51.19% <10.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
mmdet3d/models/dense_heads/point_rpn_head.py	26.79% <10.00%> (-0.23%)	⬇️
mmdet3d/datasets/pipelines/transforms_3d.py	88.26% <0.00%> (+0.85%)	⬆️

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