adding a beta parameter to the smooth_l1 loss fn (#44433)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44433
Not entirely sure why, but changing the type of beta from `float` to `double in autocast_mode.cpp and FunctionsManual.h fixes my compiler errors, failing instead at link time
fixing some type errors, updated fn signature in a few more files
removing my usage of Scalar, making beta a double everywhere instead
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D23636720
Pulled By: bdhirsh
fbshipit-source-id: caea2a1f8dd72b3b5fd1d72dd886b2fcd690af6d
