[PyTorch] IValue(const c10::Scalar&) improvements
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72551
Don't delegate to `operator=` for construction. Catch hypothetical addition of a new Scalar type via debug assertion rather than checking in prod.
Differential Revision: [D34090560](https://our.internmc.facebook.com/intern/diff/D34090560/)
Approved by: https://github.com/malfet
