Fix __irshift__ dispatch (#21047)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21047
ghimport-source-id: 8c781c9882eebb07325a1fc7aa6f340bbec18886
Differential Revision: D15529160
Pulled By: li-roy
fbshipit-source-id: d9a444e42df5c509ae10849ba6f8006fbec830c5
