Always detach tensor in visualization

Summary:
Thanks for your contribution!.
Fixed the following runtime Error : numpy() fails when visualizing an output element of a input batch during inference

Pull Request resolved: #3400

Reviewed By: jonmorton

Differential Revision: D30560943

Pulled By: ppwwyyxx

fbshipit-source-id: abe94a12b67a5c8bad2a60fff989072f99b70d16

Author: harishsg99

detectron2/utils/visualizer.py

@@ -1177,7 +1177,7 @@ def _convert_boxes(self, boxes):
         Convert different format of boxes to an NxB array, where B = 4 or 5 is the box dimension.
         """
         if isinstance(boxes, Boxes) or isinstance(boxes, RotatedBoxes):
-            return boxes.tensor.numpy()
+            return boxes.tensor.detach().numpy()
         else:
             return np.asarray(boxes)