Fix bug in HRSCDataset with clasesswise=ture

[zytx121]

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Motivation

resolve #173

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.

[yangxue0827]

LGTM

[codecov-commenter]

Codecov Report

Merging #175 (b36dda0) into dev (96de045) will increase coverage by 0.10%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##              dev     #175      +/-   ##
==========================================
+ Coverage   28.20%   28.30%   +0.10%     
==========================================
  Files         108      108              
  Lines        6967     6967              
  Branches     1048     1048              
==========================================
+ Hits         1965     1972       +7     
+ Misses       4933     4927       -6     
+ Partials       69       68       -1     

Flag	Coverage Δ
unittests	28.29% <0.00%> (+0.08%)	⬆️

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

Impacted Files	Coverage Δ
mmrotate/datasets/hrsc.py	16.80% <0.00%> (ø)
mmrotate/datasets/builder.py	51.85% <0.00%> (+25.92%)	⬆️

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