Fix empty tensor for unique_dim (#19000)
Summary:
Fixes: #18408
cc: zasdfgbnm
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19000
Reviewed By: ezyang
Differential Revision: D15470136
Pulled By: VitalyFedyunin
fbshipit-source-id: daf71566b4dbdc91927d164f813b5ee8645af1a2
