Update training_args.py to fix runtime error
Author: sahel-sh
Pull request: yes

Fix File "..../transformers/training_args.py", line 1544, in __post_init__
      for k, v in self.fsdp_config.items():
  RuntimeError: dictionary keys changed during iteration

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Comments:
Hey! Thanks for opening a PR 🤗 could you also share a reproducer? Would help determine whether this is an intended use case or not! 


From [FastChat](https://github.com/lm-sys/FastChat/blob/main/scripts/test_readme_train.sh) repo:
```
torchrun --nproc_per_node=4 --master_port=20001 fastchat/train/train_mem.py \
    --model_name_or_path lmsys/vicuna-7b-v1.5 \
    --data_path data/dummy_conversation.json \
    --bf16 True \
    --output_dir output_vicuna \
    --num_train_epochs 3 \
    --per_device_train_batch_size 2 \
    --per_device_eval_batch_size 2 \
    --gradient_accumulation_steps 16 \
    --evaluation_strategy "no" \
    --save_strategy "steps" \
    --save_steps 1200 \
    --save_total_limit 10 \
    --learning_rate 2e-5 \
    --weight_decay 0. \
    --warmup_ratio 0.03 \
    --lr_scheduler_type "cosine" \
    --logging_steps 1 \
    --fsdp "full_shard auto_wrap" --fsdp_config $HOME/fsdp_config.json \
    --tf32 True \
    --model_max_length 4096 \
    --gradient_checkpointing True \
    --lazy_preprocess True
```

Where fsdp_config.json is:
```
{
    "fsdp_auto_wrap_policy": "FULL_SHARD",
    "fsdp_transformer_layer_cls_to_wrap": "LlamaDecoderLayer"
}
```
Any command with this config file or any other config files that has json keys starting with "fsdp_" will reproduce the issue. The code is updating dictionary elements while iterating over it.

I think https://github.com/huggingface/transformers/pull/25920 is a better solution though.

Closing this in favor of https://github.com/huggingface/transformers/pull/25920