fix hardsigmoid_backward for boundary case (#51454)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/51438.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51454
Reviewed By: mruberry
Differential Revision: D26243461
Pulled By: ngimel
fbshipit-source-id: 7d954dc47427f02b7cbf0344e9889db223bfb525
