[BE] Improve test_transformers test structure (#105938)
# Summary
We have a vast majority of test that only run on cuda. Decorating with @onlycuda causes pytest to instantiate 2x the tests and skip half of them. This overhead is non trivial when the #tests cross larger like it has for this file.
This breaks up the cuda only tests into a separate class
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105938
Approved by: https://github.com/mikaylagawarecki, https://github.com/malfet
