[DDP] Make _ReplicateState inherit from _State and make replicate eagerly initialized (#109647)
Follow how fully_shard store the _FSDPState, this PR makes _ReplicateState inherit from _State. This PR also makes replicate eagerly initialize the internal DDP instance so that users can access the required methods/functions before the first forward().
Differential Revision: [D49428291](https://our.internmc.facebook.com/intern/diff/D49428291/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/109647
Approved by: https://github.com/wz337, https://github.com/rohan-varma
ghstack dependencies: #110688
