[FSDP][BE] Move dynamo annotation to separate file (#89890)
This PR makes two minor changes: It (1) moves the recently-added module annotation logic for dynamo support to a separate file `torch/distributed/fsdp/_dynamo_utils.py` and ~~(2) saves the annotated attribute names to global variables `FSDP_MANAGED_MODULE` and `FSDP_USE_ORIG_PARAMS`~~.
Update: Since the distributed package may not be included in some builds, it is not safe to import from `torch.distributed...` to a file in `_dynamo/`. I will not include change (2) in this PR. The alternative is to define those globals (privately) in the dynamo file and import from there in the FSDP file.
- The first change is mainly a personal choice, where I wanted to avoid the dynamo explanation from dominating the FSDP constructor space-wise. I added the `(see function for details)` to the inline comment to forward interested readers.
- The second change follows the custom we have taken in the past for such attributes (e.g. `FSDP_FLATTENED`). My understanding (in the past as well as currently) is that this is a good practice.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89890
Approved by: https://github.com/wconstab
