Update COCO dataset and COCODetection to detr with SplitMix class

[Johansmm]

Update COCO detection with SplitMix class.

[thibo73800]

I'm not sure it is possible right now since splitMix is used for datasets with a Split while Coco dataset is file based (ann, img_folder)

[Johansmm]

I will work in a proposition and will make a pull request with the considerations. We could discuss then.