[ET][Portable] Introduce advanced index util

Pull Request resolved: #723

Refactor `index.Tensor` helper functions into `advanced_index_util`, to be reused by `index_put`
ghstack-source-id: 203414284
@exported-using-ghexport

Differential Revision: [D50031047](https://our.internmc.facebook.com/intern/diff/D50031047/)

Author: manuelcandales