Explicitly set nn.Module.set_extra_state return type to None (#120161)
Implicitly, the return type of `set_extra_state` is `NoReturn` since it always raises an error, and pyright will complain about mismatched return types if you override it with an implementation that doesn't also always raise an error. If we explicitly hint the return type as `None` (how we expect people to override it), we can avoid this error message.
```
Method "set_extra_state" overrides class "Module" in an incompatible manner
Return type mismatch: base method returns type "NoReturn", override returns type "None"
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120161
Approved by: https://github.com/mikaylagawarecki
