Update docstring for get_model_state_dict (mosaicml#3318)

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

composer/checkpoint/state_dict.py

@@ -32,7 +32,7 @@ def get_model_state_dict(
     Args:
         model: The model to get the state dict from.
         sharded_state_dict: Whether the model state dict should be sharded or not. If True, every rank returns the state dict of its shards.
-            If False, then rank 0 returns the state dict of the entire model and the other ranks return a dict of their shards. Default is False.
+            If False, then rank 0 returns the state dict of the entire model and the other ranks return an empty dict. Default is False.
         precision: The precision of the model. Can be specified as a string ('fp32', 'fp16', 'bf16') or a torch.dtype.
         include_keys: The list of keys to exclusively include in the state dict. If None, all keys are included. Both include_keys and ignore_keys cannot be non-None.
         ignore_keys: The list of keys to ignore in the state dict. If None, no keys are ignored. Both include_keys and ignore_keys cannot be non-None.