[DDP] remove backend constraints on uneven input tests (#56754)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56754
these tests are backend agnostic and shouldn't require a specific
backend(s) to run properly. Hence enabling them regardless of the backends that
are available.
ghstack-source-id: 127463147
Test Plan: CI
Reviewed By: SciPioneer
Differential Revision: D27954174
fbshipit-source-id: 24759486b0c0647a5c88da4721a9a78d78c0b1f6
