Step 4: add support for unique with dim=None (#18651)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18651
ghimport-source-id: e11988130a3f9a73529de0b0d08b4ec25fbc639c
Differential Revision: D15000463
Pulled By: VitalyFedyunin
fbshipit-source-id: 9e258e473dea6a3fc2307da2119b887ba3f7934a
