add an embedding bag skeleton operators (#60491)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60491
basic reference embedding bag operators, these are not going to be performant but can be used for functionality enablement
these operators will output the right shape, but the implementation is empty
Test Plan: tbd
Reviewed By: vkuzo
Differential Revision: D29313058
fbshipit-source-id: b05df6ff9a7c0c6ac46ef64a42464988453bd460
