call _upcast to consider overflow for computing giou loss (#5685)
call _upcast to consider overflow for computing giou loss (#5685)

* call _upcast to consider overflow

giou loss is weak at overflow problem because it computes area of box.

* cast datatype to float

* add ":" to if

* lint Test for membership should be 'not in' (E713)

pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision