Allow FSDP to have ignored modules out of wrapped root (#91079)
Motivations for this change:
1. TorchRec returns inconsistent results on `m.named_parameters()`
and `m.m1.named_parameters()` if m1 is a `ShardedModule`. Basically,
`ShardedModule` appears in `m.named_modules()`, but its parameters
are not in `m.named_parameters()`. As a result, when we identify
`ShardedModule` and pass them as `ignored_modules` to FSDP, FSDP
complains about key error in `_get_ignored_params`.
2. If users are manually wrapping submodules with FSDP, it could be
easier for them to keep a global set of ignored parameters, instead
of create a new collection for every FSDP invocation.
Given the above two reasons, we allow FSDP to have ignored modules
out of the wrapped root module.
Differential Revision: [D42132394](https://our.internmc.facebook.com/intern/diff/D42132394)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91079
Approved by: https://github.com/awgu
