Fix module not registered bug

[Xiangxu-0103]

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Motivation

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Modification

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BC-breaking (Optional)

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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

Merging #1625 (b223f7b) into dev (b1ea1a8) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev    #1625   +/-   ##
=======================================
  Coverage   50.15%   50.15%           
=======================================
  Files         223      223           
  Lines       19124    19124           
  Branches     3141     3141           
=======================================
  Hits         9591     9591           
  Misses       8953     8953           
  Partials      580      580           

Flag	Coverage Δ
unittests	50.15% <100.00%> (ø)

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Impacted Files	Coverage Δ
mmdet3d/models/roi_heads/__init__.py	100.00% <100.00%> (ø)

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