Remove dead ScatterGather code (#39963)
Summary:
This code was probably left behind after an ATen port.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39963
Differential Revision: D22039370
Pulled By: ezyang
fbshipit-source-id: 4ef75bac9b69f4b508a0b09c5c1f2ebc21bd9546
