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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

Modification

I changed the hardcoded 3 channel length to dynamic channel length innp.full function arguments.
This modification enablesRandomMosaic transform to support multispectral image (e.g. RGB image with NIR band) or bi-temporal image pairs for change detection task.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-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. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMDet3D.
4. The documentation has been modified accordingly, like docstring or example tutorials.