[WIP] Add PolyRandomAffine and FilterRotatedAnnotations

[liuyanyi]

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Motivation

Add PolyRandomAffine and FilterRotatedAnnotations.
Part of RotatedYOLOX.

For the rect_classes in PolyRandomRotate, I didn't find a better solution to add them to the RandomAffine, so I ignored it for now.

I think there may be a problem with clip_border too. This may change the bbox shape at edges.

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.

[liuyanyi]

Should I rename that to PolyRandomAffine, the transform is done in polygon.

[codecov]

Codecov Report

Merging #353 (dcde999) into dev (36de5f6) will decrease coverage by 0.20%.
The diff coverage is 16.82%.

@@            Coverage Diff             @@
##              dev     #353      +/-   ##
==========================================
- Coverage   29.46%   29.25%   -0.21%     
==========================================
  Files         121      121              
  Lines        8495     8596     +101     
  Branches     1289     1303      +14     
==========================================
+ Hits         2503     2515      +12     
- Misses       5891     5980      +89     
  Partials      101      101              

Flag	Coverage Δ
unittests	29.23% <16.82%> (-0.21%)	⬇️

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

Impacted Files	Coverage Δ
mmrotate/datasets/pipelines/transforms.py	52.02% <12.16%> (-13.76%)	⬇️
mmrotate/core/bbox/transforms.py	36.07% <16.66%> (-0.25%)	⬇️
mmrotate/datasets/pipelines/loading.py	25.53% <20.83%> (-4.91%)	⬇️
mmrotate/core/bbox/__init__.py	100.00% <100.00%> (ø)
mmrotate/datasets/pipelines/__init__.py	100.00% <100.00%> (ø)

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[zytx121]

Should I rename that to PolyRandomAffine, the transform is done in polygon.

Yes.

[liuyanyi]

Continue in #409