[Enhance] use iof for RRandomCrop validation

[nijkah]

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Motivation

Previously, valid_rbboxes are determined with their center points.
Following

mmrotate/tools/data/dota/split/img_split.py

Line 79 in bc1ced4

'--iof-thr',

, replace it with more appropriate criteria: IoF(Intersection over foreground)

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.

[zytx121]

I think it's reasonable.

[codecov]

Codecov Report

Base: 29.44% // Head: 29.48% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (bdf0c6f) compared to base (bc1ced4).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #660      +/-   ##
==========================================
+ Coverage   29.44%   29.48%   +0.03%     
==========================================
  Files         121      121              
  Lines        8500     8504       +4     
  Branches     1279     1279              
==========================================
+ Hits         2503     2507       +4     
  Misses       5896     5896              
  Partials      101      101              

Flag	Coverage Δ
unittests	29.45% <100.00%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
mmrotate/datasets/pipelines/transforms.py	66.37% <100.00%> (+0.59%)	⬆️

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