[Composable API][Easy] Use `policy=None` since that is supported (#90400)
I believe that @mrshenli used `ModuleWrapPolicy({UnitModule})` when applying `fully_shard` to `UnitModule`s because `policy=None` was not supported. However, he added that support in a previous PR, so this PR simplifies to using `policy=None` to make the intention more clear.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90400
Approved by: https://github.com/mrshenli
