porting scatter_add to ATen (CPU) (#31662)
Summary:
Fixes [https://github.com/pytorch/pytorch/issues/24758](https://github.com/pytorch/pytorch/issues/24758).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31662
Differential Revision: D19440824
Pulled By: ngimel
fbshipit-source-id: b13443cfcc8bcb9ec21f1cddb5c6fbc0ef4bb0f2
