Fix PonitRCNN GPU memory occupancy problem

[VVsssssk]

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Motivation

Fix PonitRCNN GPU memory occupancy problem. Issue #1907

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the back-compatibility of the downstream repos?
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.

[VVsssssk]

issue #1907

[codecov]

Codecov Report

Base: 49.96% // Head: 49.96% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (d55dbdb) compared to base (b9f6e62).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1928   +/-   ##
=======================================
  Coverage   49.96%   49.96%           
=======================================
  Files         228      228           
  Lines       19915    19915           
  Branches     3187     3187           
=======================================
+ Hits         9950     9951    +1     
+ Misses       9375     9374    -1     
  Partials      590      590           

Flag	Coverage Δ
unittests	49.96% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...odels/roi_heads/bbox_heads/point_rcnn_bbox_head.py	30.31% <100.00%> (+0.13%)	⬆️
mmdet3d/models/roi_heads/point_rcnn_roi_head.py	26.66% <100.00%> (+0.70%)	⬆️

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[ZCMax]

please fix the ut