[BE][FSDP] Introduce `FSDPTestModel` interface (#80873)
**Overview**
Please refer to https://github.com/pytorch/pytorch/issues/80867 first.
This addresses:
> Goal 3: Refactor model construction to enable simpler testing for the non-recursive wrapping path.
The idea is that we have an abstract class `FSDPTestModel` that defines the interface expected from the parity check and training boilerplate. This PR refactors the models in `common_fsdp.py` used in `test_fsdp_core.py` to implement this interface. Further unification under this interface is coming in follow-up PRs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80873
Approved by: https://github.com/rohan-varma
