[Composable API] Make _StateKey as a str subclass (#91279)
The keys in object.__dict__ should be strings. Make the _StateKey be a str subclass.
Differential Revision: [D42200244](https://our.internmc.facebook.com/intern/diff/D42200244/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91279
Approved by: https://github.com/awgu, https://github.com/mrshenli
