Fix bug when training images without gt.

[zytx121]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Fix bug when training images without gt.

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]

Codecov Report

Patch coverage has no change and project coverage change: +17.65 🎉

Comparison is base (c1aa0f9) 61.68% compared to head (50d3ccf) 79.33%.

Additional details and impacted files

@@             Coverage Diff              @@
##           dev-1.x     #786       +/-   ##
============================================
+ Coverage    61.68%   79.33%   +17.65%     
============================================
  Files          100      100               
  Lines         7269     7269               
  Branches      1040     1040               
============================================
+ Hits          4484     5767     +1283     
+ Misses        2534     1164     -1370     
- Partials       251      338       +87     

Flag	Coverage Δ
unittests	?

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

Impacted Files	Coverage Δ
mmrotate/structures/bbox/bbox_overlaps.py	94.87% <0.00%> (ø)

... and 30 files with indirect coverage changes

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.