Bug fix: RuntimeError: Expected all tensors to be on the same device,…

… but found at least two devices, cuda:0 and cuda:2! (#7521)

修复Free anchor Retinanet网络训练时，可能会遇到计算得到的anchor与pre_bboxes不在同一个cuda device的bug。

mmdet/models/dense_heads/free_anchor_retina_head.py

@@ -78,8 +78,8 @@ def loss(self,
         """
         featmap_sizes = [featmap.size()[-2:] for featmap in cls_scores]
         assert len(featmap_sizes) == self.prior_generator.num_levels
-
-        anchor_list, _ = self.get_anchors(featmap_sizes, img_metas)
+        device = cls_scores[0].device
+        anchor_list, _ = self.get_anchors(featmap_sizes, img_metas, device=device)
         anchors = [torch.cat(anchor) for anchor in anchor_list]
 
         # concatenate each level