Introduce advanced index util (#723)

Summary:
Pull Request resolved: #723

Refactor `index.Tensor` helper functions into `advanced_index_util`, to be reused by `index_put`
ghstack-source-id: 203418804
exported-using-ghexport

Reviewed By: SS-JIA

Differential Revision: D50031047

fbshipit-source-id: e6ba5d35e8b5efca03dc4a11a813574c469cf937

Author: manuelcandales