Update ATen/native/README.md about broadcasting (#29742)
Summary:
Is this description still true? I have never seen any `s_` ops.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29742
Differential Revision: D18485707
Pulled By: ezyang
fbshipit-source-id: c5ce2587bb499561706c3c2817571ee11f7eb63c
