Fix loss_bbox in fovea_head.py when num_pos=0 (#2548)

* fix bbox_loss in fovea_head.py when num_pos=0

* Remove PIL from .isort.cfg

* Make loss_bbox scalar in fovea_head.py when num_pos==0

* Sum based reduction for loss_bbox in fovea_head

* reverting to scalar implementation

.isort.cfg

@@ -3,6 +3,6 @@ line_length = 79
 multi_line_output = 0
 known_standard_library = setuptools
 known_first_party = mmdet
-known_third_party = asynctest,cityscapesscripts,cv2,matplotlib,mmcv,numpy,onnx,pycocotools,robustness_eval,roi_align,roi_pool,seaborn,six,terminaltables,torch,torchvision
+known_third_party = PIL,asynctest,cityscapesscripts,cv2,matplotlib,mmcv,numpy,onnx,pycocotools,robustness_eval,roi_align,roi_pool,seaborn,six,terminaltables,torch,torchvision
 no_lines_before = STDLIB,LOCALFOLDER
 default_section = THIRDPARTY

mmdet/models/anchor_heads/fovea_head.py

@@ -224,9 +224,10 @@ class FoveaHead(nn.Module):
                 pos_weights,
                 avg_factor=num_pos)
         else:
-            loss_bbox = torch.tensor([0],
-                                     dtype=flatten_bbox_preds.dtype,
-                                     device=flatten_bbox_preds.device)
+            loss_bbox = torch.tensor(
+                0,
+                dtype=flatten_bbox_preds.dtype,
+                device=flatten_bbox_preds.device)
         return dict(loss_cls=loss_cls, loss_bbox=loss_bbox)
 
     def fovea_target(self, gt_bbox_list, gt_label_list, featmap_sizes, points):