Generalize masks_to_boxes op to support N-dimensional masks to bounding boxes conversion

[RylanSchaeffer]

🚀 The feature

Ops to convert N-dimensional segmentation masks to N-dimensional bounding boxes.

Motivation, pitch

Build upon #3960 to support the conversion of N-dimensional asks to bounding boxes.

Use cases:

1. I have 3D images (from medical imaging) and would like to convert segmentation masks (of tumors, aneurysms, etc.) to bounding boxes.

2. Sometimes different sources of data are stacked together, giving me 4D images (3D images, stacked along a new dimension to make the image source clear). I would also like to convert these segmentation masks to bounding boxes, although I will admit, I would compute bounding boxes in 3D and then stack those as well.