Fix potential bug of build roi_extractors

[Xiangxu-0103]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

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)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

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 #1460 (ba2a406) into dev (4422eaa) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##              dev    #1460      +/-   ##
==========================================
+ Coverage   50.39%   50.42%   +0.03%     
==========================================
  Files         221      221              
  Lines       18927    18927              
  Branches     3118     3118              
==========================================
+ Hits         9538     9544       +6     
+ Misses       8811     8806       -5     
+ Partials      578      577       -1     

Flag	Coverage Δ
unittests	50.42% <0.00%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
mmdet3d/models/builder.py	84.93% <0.00%> (ø)
mmdet3d/datasets/pipelines/test_time_aug.py	51.04% <0.00%> (ø)
mmdet3d/datasets/pipelines/transforms_3d.py	88.25% <0.00%> (+0.85%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4422eaa...ba2a406. Read the comment docs.