containing:
- boxes (Tensor[N, 4]): the ground-truth boxes in [x0, y0, x1, y1] format, with values
between 0 and H and 0 and W
- labels (Tensor[N]): the class label for each ground-truth box
- keypoints (Tensor[N, K, 3]): the K keypoints location for each of the N instances, in the
format [x, y, visibility], where visibility=0 means that the keypoint is not visible.

The model returns a Dict[Tensor] during training, containing the classification and regression<