[Fix] fix coco registry problem

[ZCMax]

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Motivation

fix error: KeyError: 'CocoDataset is not in the dataset registry', more details can be found in #1531.

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.

[Tai-Wang]

Only need to merge the latest dev to check CI when #1537 is merged.

[codecov]

Codecov Report

Merging #1536 (c2865bc) into dev (be8f653) will decrease coverage by 0.00%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##              dev    #1536      +/-   ##
==========================================
- Coverage   50.16%   50.15%   -0.01%     
==========================================
  Files         222      222              
  Lines       19092    19095       +3     
  Branches     3132     3133       +1     
==========================================
+ Hits         9577     9578       +1     
- Misses       8938     8939       +1     
- Partials      577      578       +1     

Flag	Coverage Δ
unittests	50.15% <33.33%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
mmdet3d/datasets/builder.py	42.42% <33.33%> (-0.91%)	⬇️

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