[pytorch] Migrating index_add cuda to ATen (#30573)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30573
Mostly just moved code.
Index dim and number of indices checks are added to make checks idential to index_add_cpu_
ghstack-source-id: 98010129
Test Plan: existing tests
Differential Revision: D18749922
fbshipit-source-id: d243be43a3b6a9b9591caf0c35ef2fb6ec0d3ead
