[FSDP][8/N] Replace `_FSDPPolicy.policy` with `_Policy._run_policy` (#104969)
This does some code organization improvement.
- It renames `_FSDPPolicy` to `_Policy` to show that it is not only for FSDP but for any module-level API.
- It formalizes the contract that such a policy should return something like `target_module_to_kwargs: Dict[nn.Module, Dict[str, Any]]` that maps each module to wrap to its kwargs. It does so by requiring a `_run_policy` abstract method (this time private since users do not need to care about it). Then, our auto wrapping can just call `_run_policy()` to generate the dict and do any validation or post-processing.
This PR is technically BC-breaking because it removes the public `ModuleWrapPolicy.policy`. However, I do not think anyone was using that anyway, so this is a pretty safe breakage.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/104969
Approved by: https://github.com/rohan-varma
ghstack dependencies: #104427, #104967, #104999
