Add scatter_add to amp docs (#54908)
Summary:
Updates docs to reflect https://github.com/pytorch/pytorch/pull/52133.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54908
Reviewed By: agolynski
Differential Revision: D27431302
Pulled By: H-Huang
fbshipit-source-id: fa3dc6267bc73c81cdd96f986c971daee1922cb5
