[Fix] Fix nuscenes_mono_dataset unit test due to hack fix

[Tai-Wang]

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Motivation

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Fix nuscenes_mono_dataset unit test due to #744 .

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 #795 (52a5c4b) into master (a8f4752) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #795   +/-   ##
=======================================
  Coverage   48.94%   48.94%           
=======================================
  Files         208      208           
  Lines       15884    15884           
  Branches     2538     2538           
=======================================
  Hits         7774     7774           
  Misses       7608     7608           
  Partials      502      502           

Flag	Coverage Δ
unittests	48.94% <ø> (ø)

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