Support ADE20k for semantic segmentation with deeplabv3+

## 🚀 Feature
Support ADE20k for semantic segmentation with deeplabv3+

## Motivation & Examples

This will be helpful when people need to benchmark different self-supervised learning method, since most ssl paper do experiments on ade20k semantic segmantation dataset.

For example: the repo of [moco](https://github.com/facebookresearch/moco) uses detectron2 to do experiments on downstream task of object detection, but it lacks semantic segmantation experiments. Maybe it is because detectron2 does not support it directly and explicitly.