deepspeed train flux1 dreambooth lora can not save model

### Describe the bug

when I run the script train_dreambooth_lora_flux.py. It raise ValueError: unexpected save model: . something bug in save_model_hook?
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### Reproduction

accelerate launch train_dreambooth_lora_flux_custom.py \
--pretrained_model_name_or_path=$MODEL_NAME \
--instance_data_dir=$INSTANCE_DIR \
--output_dir=$OUTPUT_DIR \
--mixed_precision="bf16" \
--instance_prompt="bedroom, YF_CN style" \
--resolution=1024 \
--train_batch_size=1 \
--guidance_scale=1 \
--gradient_accumulation_steps=4 \
--optimizer="prodigy" \
--learning_rate=1. \
--report_to="tensorboard" \
--lr_scheduler="constant" \
--lr_warmup_steps=0 \
--num_train_epochs=30 \
--validation_prompt="bedroom, YF_CN style" \
--validation_epochs=80 \
--checkpointing_steps=500 \
--seed="0" \
--gradient_checkpointing \
--use_8bit_adam \
--rank=4

### Logs

_No response_

### System Info

torch==2.3.1
accelerate==0.34.2
deepspeed==0.15.1+8ac42ed7
diffusers==0.31.0.dev0

default_config.yaml as follow:

compute_environment: LOCAL_MACHINE
debug: true
deepspeed_config:
gradient_accumulation_steps: 1
gradient_clipping: 1.0
offload_optimizer_device: none
offload_param_device: none
zero3_init_flag: false
zero_stage: 2
distributed_type: DEEPSPEED
downcast_bf16: 'no'
enable_cpu_affinity: false
machine_rank: 0
main_training_function: main
mixed_precision: fp16
num_machines: 1
num_processes: 1
rdzv_backend: static
same_network: true
tpu_env: []
tpu_use_cluster: false
tpu_use_sudo: false
use_cpu: fals

### Who can help?

@sayakpaul

Start by reproducing the command with train_dreambooth_lora_flux.py and the listed Accelerate/DeepSpeed configuration, then inspect the save_model_hook path involved in checkpoint saving. Compare how the DeepSpeedEngine is handled during saving; done means the Flux DreamBooth LoRA training run saves the model without the unexpected save model ValueError.

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