[DDP] Refactor _DDPSink to take DDP weakref (#103304)
This will make future PRs to support DDP static graph multi forward
cleaner.
Differential Revision: [D46584545](https://our.internmc.facebook.com/intern/diff/D46584545/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103304
Approved by: https://github.com/awgu
