Move lerp to ATen, add functionality for tensor weights (#17348)
Summary:
Changelog:
- Remove TH/THC bindings
- Add tensor weights for `lerp`
- Modify derivatives appropriately
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17348
Differential Revision: D14355845
Pulled By: soumith
fbshipit-source-id: eaede4c09ee589d77ba6cf52583510ea8e3a2fcf
