[Fix] Fix the error of data type in waymo metric

[JingweiZhang12]

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Motivation

Now, after evaluation on Waymo, the type of results is dict(dict). It's not supported in MMEngine. It should be a dict.

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.

[codecov]

Codecov Report

Base: 45.28% // Head: 45.28% // No change to project coverage 👍

Coverage data is based on head (19c7715) compared to base (d7067e4).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           dev-1.x    #2109   +/-   ##
========================================
  Coverage    45.28%   45.28%           
========================================
  Files          255      255           
  Lines        20825    20825           
  Branches      3289     3289           
========================================
  Hits          9431     9431           
  Misses       10774    10774           
  Partials       620      620           

Flag	Coverage Δ
unittests	45.28% <0.00%> (ø)

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

Impacted Files	Coverage Δ
mmdet3d/evaluation/metrics/waymo_metric.py	8.14% <0.00%> (ø)

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