clean up test_cat_empty (#28115)
Summary:
Remove spurious parts from test_cat_empty
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28115
Test Plan: no additional tests needed.
Differential Revision: D17956669
Pulled By: ngimel
fbshipit-source-id: cffcfa9e5b50afba62c6dbc8ca5d9de95d0c020e
