[DDP] Rename state_dict var to ddp_state (#103282)
This name is confusing in the context that it is just a dictionary
used to pass state to DDP backward pass.
Differential Revision: [D46580516](https://our.internmc.facebook.com/intern/diff/D46580516/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103282
Approved by: https://github.com/awgu
