[Fix] fix visualization bugs

[jbwang1997]

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Motivation

Fix #9
Fix the visualization bugs brought by the new feature in open-mmlab/mmdetection#6716.

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. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov-commenter]

Codecov Report

Merging #12 (43d724c) into main (6519a36) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #12   +/-   ##
=======================================
  Coverage   28.92%   28.92%           
=======================================
  Files          97       97           
  Lines        6471     6471           
  Branches      969      969           
=======================================
  Hits         1872     1872           
  Misses       4534     4534           
  Partials       65       65           

Flag	Coverage Δ
unittests	28.92% <100.00%> (ø)

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

Impacted Files	Coverage Δ
mmrotate/models/detectors/base.py	25.00% <ø> (ø)
mmrotate/core/visualization/image.py	77.77% <100.00%> (ø)

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[zytx121]

LGTM.