[Enhance] Support multi-band image for Mosaic (#2748)

## Modification

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

## 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.

mmseg/datasets/transforms/transforms.py

@@ -1062,8 +1062,9 @@ class RandomMosaic(BaseTransform):
 
         assert 'mix_results' in results
         if len(results['img'].shape) == 3:
+            c = results['img'].shape[2]
             mosaic_img = np.full(
-                (int(self.img_scale[0] * 2), int(self.img_scale[1] * 2), 3),
+                (int(self.img_scale[0] * 2), int(self.img_scale[1] * 2), c),
                 self.pad_val,
                 dtype=results['img'].dtype)
         else:

tests/test_datasets/test_transform.py

@@ -639,6 +639,17 @@ def test_mosaic():
     results = mosaic_module(results)
     assert results['img'].shape[:2] == (20, 24)
 
+    results = dict()
+    results['img'] = np.concatenate((img, img), axis=2)
+    results['gt_semantic_seg'] = seg
+    results['seg_fields'] = ['gt_semantic_seg']
+
+    transform = dict(type='RandomMosaic', prob=1, img_scale=(10, 12))
+    mosaic_module = TRANSFORMS.build(transform)
+    results['mix_results'] = [copy.deepcopy(results)] * 3
+    results = mosaic_module(results)
+    assert results['img'].shape[2] == 6
+
 
 def test_cutout():
     # test prob