[Executorch][custo ops] Add prototype defs for custom op (#9815)

Pull Request resolved: https://github.com/pytorch/executorch/pull/9786

To fix mac buck builds
ghstack-source-id: 275283538

Differential Revision:
[D71370605](https://our.internmc.facebook.com/intern/diff/D71370605/)

Co-authored-by: Kimish Patel <kimishpatel@fb.com>