Update docstring for get_model_state_dict

[eracah]

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Turns out it's empty dict for nonzero ranks for unsharded state dicts because for torch 2.1.2 we set the FullStateDictConfig rank0_only flag to True and for torch >2.1.2, the dcp.get_model_state_dict function always has empty dict for nonzero ranks for unsharded state dicts