Batched box scale

🚀 The feature

Frequent utility in postprocessing box detections:

def batched_box_scale(boxes, image_shape, target_image_shape):
  fx = target_image_shape[..., -1] / image_shape[...., -1]
  fy = target_image_shape[..., -2] / image_shape[..., -2]
  return boxes * torch.stack([fx, fy, fx, fy], dim = -1).to(boxes.device)

Motivation, pitch

This is a frequently needed utility to bring back computed detections to a common / original coordinate frame.

cc @vfdev-5 @datumbox

[fmassa]

I meant this will be handled by

# boxes is a custom object that is defined in #4721
boxes : BBox

# then the transforms will know how to resize them
T.resize(boxes, (50, 20))

[fmassa]

@vadimkantorov new abstractions with transforms are in #4861

I hope they will be useful and would encourage users to use them