add fill_diagonal function (#21892)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/21796
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21892
Differential Revision: D16164678
Pulled By: colesbury
fbshipit-source-id: 85df8ae9b7a6a91b6023fe7295b3a8124e4526ea
