[dtensor][4/N] refactor dispatching logic and add propagator (#90733)
This PR refactors the dispatching logic to make it more clean, and
isolate the sharding propagation logic out to a separate class.
This is so that we can implement more complicated propagation features
later.
Differential Revision: [D42876251](https://our.internmc.facebook.com/intern/diff/D42876251)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90733
Approved by: https://github.com/XilunWu, https://github.com/fduwjj
