update dp_rank to rdp_rank for opt_state_dict

This PR fixes hang for SMP when calling state_dict(). 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15804). All of your documentation changes will be reflected on that endpoint.<|||||>When only pipeline parallelism is used, dp_rank works. But with newer features of SageMaker model parallel library we support Tensor parallelism too. Tensor parallelism is done within a DP group. In such a case, we need to save the state on all ranks with rdp_rank=0. Essentially two ranks in an RDP group have copies of the same shard of parameter. More details on ranking mechanism here https://docs.aws.amazon.com/sagemaker/latest/dg/model-parallel-extended-features-pytorch-ranking-mechanism.html
<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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