[optim][fix] test_fused_optimizers did not test fused before (#91228)
I realized test_fused_optimizers used a helper that was written for foreach, so we were not testing fused at all. This PR fixes that test so we actually test fused adam.
The explicitly adding fused=False is to set the stage for my later changes (but should be a no-op here).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91228
Approved by: https://github.com/albanD, https://github.com/soulitzer
