[Fix] Fix cd transform

[Zoulinx]

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Motivation

Fix the bug that data augmentation only takes effect on one image in the change detection task.

Modification

configs/base/datasets/levir_256x256.py
configs/swin/swin-tiny-patch4-window7_upernet_1xb8-20k_levir-256x256.py
mmseg/datasets/transforms/transforms.py

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.