feature request: Dice metric for segmentation models

The library already has [mean_iou (mean Jaccard index)](https://en.wikipedia.org/wiki/Jaccard_index) which I have used for my image segmentation models and it works well.

If you look at recent research papers on the topic of image segmentation for medical imaging, you will notice that [the Dice metric (Sorensen-Dice coefficient)](https://en.wikipedia.org/wiki/S%C3%B8rensen%E2%80%93Dice_coefficient) is used a lot in research.

It would be very useful to have the Dice metric integrated into the main library - that would simplify model comparison when reviewing the literature. Thanks!