[DDP] API to get data parallel parameters (#95097)
Add a private API to retrieve data parallel parameters. This is
useful for example for apply_optimizer_in_backward in the case user wishes to
ensure it is applied only on DDP managed parameters.
Differential Revision: [D43383878](https://our.internmc.facebook.com/intern/diff/D43383878/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95097
Approved by: https://github.com/zhaojuanmao, https://github.com/fegin
