AOTDispatch subclass (#104483)
This is a PoC of AOTDispatch support. This PR actually works on basic examples, and I'm working on testing it out on `DTensor` (with @wanchaol), `SemiStructuredSparsityTensor` (with @jcaip), and `FP8Tensor`.
There are some design decisions baked into the PR that I think we need consensus on though - so I'm planning on writing a larger design doc to go over the changes.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/104483
Approved by: https://github.com/ezyang
