Fix named tensor test (#25313)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25313
`sign` was recently ported from TH to ATen, undoing some named tensor
changes and breaking the CI named tensor test. This PR re-enables named tensor
for `sign`.
Test Plan
- [namedtensor ci]
Test Plan: Imported from OSS
Differential Revision: D17093439
Pulled By: zou3519
fbshipit-source-id: 11185ad88a0eaf56078b94e9547bbbd6d02d0aab
