More numerically stable lerp (#18871)
Summary:
The C++ and CUDA implementations of the lerp are not numerically stable. This is discussed on Wikipedia [here](https://en.wikipedia.org/wiki/Linear_interpolation#Programming_language_support). I checked the GPU SASS output and there's no overhead from using the more precise implementation, from Kepler all the way to Turing. I haven't looked at CPU ASM though.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18871
Differential Revision: D14793438
Pulled By: ezyang
fbshipit-source-id: 2ddc2e026c5285466cae7d1b4101174253100445
