Avoid undefined symbol error when building AdIndexer LTO (#19009)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19009
Move the definition of `MulFunctor<>::Backward()` into a header file.
Reviewed By: BIT-silence
Differential Revision: D14823230
fbshipit-source-id: 1efaec01863fcc02dcbe7e788d376e72f8564501
