Fix mAP calculation for areas with 0 predictions

The issue was first dicussed here: #1061 (comment)

torchmetrics/detection/mean_ap.py

@@ -482,9 +482,11 @@ def __evaluate_image_gt_no_preds(
     ) -> Dict[str, Any]:
         """Some GT but no predictions."""
         # GTs
-        gt = gt[gt_label_mask]
+        gt = np.asarray(gt)[gt_label_mask]
+        if len(gt_label_mask) == 1:
+            gt = np.expand_dims(gt, axis=0)
         nb_gt = len(gt)
-        areas = box_area(gt)
+        areas = compute_area(gt, iou_type=self.iou_type).to(self.device)
         ignore_area = (areas < area_range[0]) | (areas > area_range[1])
         gt_ignore, _ = torch.sort(ignore_area.to(torch.uint8))
         gt_ignore = gt_ignore.to(torch.bool)