[FSDP][Dynamo] Define annotation attributes as globals (#89913)
This was separated out from the previous PR to decouple. Since not all builds include `torch.distributed`, we should define the globals in the dynamo file and import to distributed instead of vice versa. Unlike the version from the previous PR, this PR prefixes the globals with `_` to future proof against `_dynamo/` eventually becoming public.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89913
Approved by: https://github.com/wconstab
