remove scripting hack for classmethod

Summary: require pytorch/pytorch#49967

Reviewed By: theschnitz

Differential Revision: D25852122

fbshipit-source-id: eb2da41f28861fd8ac4f348283bb072abfe8fedd

detectron2/structures/boxes.py

@@ -297,14 +297,6 @@ def cat(cls, boxes_list: List["Boxes"]) -> "Boxes":
         Returns:
             Boxes: the concatenated Boxes
         """
-        if torch.jit.is_scripting():
-            # https://github.com/pytorch/pytorch/issues/18627
-            # 1. staticmethod can be used in torchscript, But we can not use
-            # `type(boxes).staticmethod` because torchscript only supports function
-            # `type` with input type `torch.Tensor`.
-            # 2. classmethod is not fully supported by torchscript. We explicitly assign
-            # cls to Box as a workaround to get torchscript support.
-            cls = Boxes
         assert isinstance(boxes_list, (list, tuple))
         if len(boxes_list) == 0:
             return cls(torch.empty(0))