Implement lerp's derivative w.r.t. weight (#28219)
Summary:
Closes https://github.com/pytorch/pytorch/issues/22444.
It seemed low priority, but the necessary change seems trivial, so I made this PR anyway.
Thanks in advance for reviewing this.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28219
Differential Revision: D17989123
Pulled By: ezyang
fbshipit-source-id: d122b50e90b63dc5d2eeb7689b5ea29d973424ed
