[Feature] Support RBox CopyPaste

[nijkah]

Motivation

Support CopyPaste when the mask is not available.
This feature is available after the PR open-mmlab/mmdetection#10509 on MMDetection is merged.

Examples

Modification

There was a bug in mask to rbox.
Since np.nonzero returns (ys, xs), it should be converted to find contours.

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]

@nijkah Nice work! I will wait for the PR open-mmlab/mmdetection#10509 on MMDetection.

[nijkah]

Hi, @zytx121. Since the PR open-mmlab/mmdetection#10509 is merged, check this PR, please. :)
Tell me if you think I have to add UT for this PR. :)

[nijkah]

Hi @zytx121 , Can I ask you to check this? :)

[dou3516]

Hi, @nijkah , How to use this CopyPaste in mmrotate? Thanks~