Update training_args.py to remove the runtime error

[sahel-sh]

This cl iterates through a list of keys rather than dict items while updating the dict elements. Fixes the following error: File "..../transformers/training_args.py", line 1544, in post_init for k, v in self.fsdp_config.items():
RuntimeError: dictionary keys changed during iteration

What does this PR do?

Fixes # (issue)

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[sahel-sh]

Steps to reproduce it:
Use a fsdp config file with json keys starting with "fsdp_", for example the config file below:
{
"fsdp_auto_wrap_policy": "FULL_SHARD",
"fsdp_transformer_layer_cls_to_wrap": "LlamaDecoderLayer"
}
The runtime error happens since the code is updating a dictionary to remove "fsdp_" from its keys while iterating over it.

[amyeroberts]

Thanks for fixing!

cc @muellerzr @pacman100

[HuggingFaceDocBuilderDev]

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