RandomMosaic not working until... (open-mmlab#1761)

RandomMosaic is not working with the suggested configuration. In order to make use of the MultiImageMixDataset, the following lines:
```
dict(type='LoadImageFromFile')
dict(type='LoadAnnotations')
```
should be provided to both the wrapped and wrapper datasets.

Author: mawanda-jun

docs/en/tutorials/customize_datasets.md

@@ -262,6 +262,8 @@ An example of using `MultiImageMixDataset` with `Mosaic` data augmentation:
 
 ```python
 train_pipeline = [
+    dict(type='LoadImageFromFile'),
+    dict(type='LoadAnnotations'),
     dict(type='RandomMosaic', prob=1),
     dict(type='Resize', img_scale=(1024, 512), keep_ratio=True),
     dict(type='RandomFlip', prob=0.5),