Migrate index_add cpu from TH to ATen (#28421)
Summary:
Migrate index_add cpu from TH to ATen.
I couldn't find replacement for get1d and set1d, so doing pointer arithmetic inplace.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28421
Test Plan: existing tests
Differential Revision: D18060971
Pulled By: ggoossen
fbshipit-source-id: 413719990cdb2fe578964cde14e93577e48a4342
