add dense vector to id_list operator (#15090)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15090
as title
step 2 of the linked task
Reviewed By: ellie-wen
Differential Revision: D13425977
fbshipit-source-id: f3538ed68f42470ba39c5b779af764d4a5591a9d
