In tvOS all buttons and cells have rounded corners and the nice parallax effect. When you use UICollectionViews you can use a UIImageView as the focusable item by setting adjustsImageWhenAncestorFocused = true. When you do this you can us tvOS ImageStacks and the effect works.
However I can't seem to find where to specifiy that the corners should be rounded? Also the example project that Apple provides uses photos with no rounded corners. You can't use layer.cornerRadius because if the ImageView's property clipsToBounds is true the effect doesn't work anymore obviously.