Improve verifier to not specialize on dialect. (#1528)

Summary:
X-link: https://github.com/pytorch/pytorch/pull/116705

Pull Request resolved: https://github.com/pytorch/executorch/pull/1528

Currently we have a very ugly specialization on edge dialect in verifier like the following:
```
 # TODO Remove this branch.
            if ep.dialect == "EDGE":  # !!! Don't change this allowlist. !!!
                pass
            else:
                raise e
```
In this diff we do some additional work to make signature checking also work in exir. We decouple the transformation stack in torch export and exir so that different layers of the stack can evolve in their own fashion and the team can divide and conquer them seperately.

Reviewed By: angelayi

Differential Revision: D52499225

fbshipit-source-id: d667397b8434d8f6b0d0d097d31a0dd589c329cc