Fix IOU assigners when ignore_of_thr > 0 and no pred boxes

[Erotemic]

When ignore_of_thr is enabled and there are no predicted bboxes, the .max function fails because the ignore_overlaps tensor is empty. The error is as follows:

~/code/mmdetection/mmdet/core/bbox/assigners/max_iou_assigner.py in assign(self, bboxes, gt_bboxes, gt_bboxes_ignore, gt_labels)
    104                 ignore_overlaps = bbox_overlaps(
    105                     bboxes, gt_bboxes_ignore, mode='iof')
--> 106                 ignore_max_overlaps, _ = ignore_overlaps.max(dim=1)
    107             else:
    108                 ignore_overlaps = bbox_overlaps(

RuntimeError: cannot perform reduction function max on tensor with no elements because the operation does not have an identity

This can be fixed by simply checking that bboxes isn't empty before running the code. I've added this fix for both approx and max iou assigners. I've also added a test, which currently passes, but will fail if the patch is removed.

[hellock]

Thanks for the fix! Since there are lots of conditions, we can just use one pair of brackets to hold them.

if (self.ignore_iof_thr > 0 and gt_bboxes_ignore is not None
    and gt_bboxes_ignore.numel() > 0 and bboxes.numel() > 0):
    pass

[hellock]

BTW, it seems that the latest ubelt (0.8.9) is broken. An import error will be raised for import ubelt.

[Erotemic]

Yes, ubelt 0.8.9 was broken. I had incorrectly used find_packages in the setup.py file. I just released 0.9.0, which (I believe) fixes the issue. I've also removed the extra parens.