AttributeError: 'str' object has no attribute 'convert' when training controlnet

### Describe the bug

when I training controlnet (by **example/controlnet** ), `AttributeError: 'str' object has no attribute 'convert' ` appears

### Reproduction

I use the same bash with example/controlnet/README.md

```
accelerate launch train_controlnet.py \
--pretrained_model_name_or_path=$MODEL_DIR \
--output_dir=$OUTPUT_DIR \
--dataset_name=fusing/fill50k \
--resolution=512 \
--learning_rate=1e-5 \
--validation_image "./conditioning_image_1.png" "./conditioning_image_2.png" \
--validation_prompt "red circle with blue background" "cyan circle with brown floral background" \
--train_batch_size=1 \
--gradient_accumulation_steps=4

```

I download **fill50K** dataset and put it in **example/controlnet**. The folder like this:

```
example/controlnet:
-- train_controlnet.py
-- conditioning_image_1.png
-- conditioning_image_2.png
-- fusing/fill50k
-- train.jsonl
-- conditioning_images
-- 0.png
-- 1.png
......
-- images
-- 0.png
-- 1.png
......
```

### Logs

```shell
Downloading data files: 100%|████████████████████████████████████████████████████████████| 1/1 [00:00<00:00, 11066.77it/s]
Extracting data files: 100%|██████████████████████████████████████████████████████████████| 1/1 [00:00<00:00, 1844.46it/s]
Generating train split: 50000 examples [00:00, 1874549.27 examples/s]
02/05/2024 20:05:51 - INFO - __main__ - ***** Running training *****
02/05/2024 20:05:51 - INFO - __main__ - Num examples = 50000
02/05/2024 20:05:51 - INFO - __main__ - Num batches each epoch = 50000
02/05/2024 20:05:51 - INFO - __main__ - Num Epochs = 1
02/05/2024 20:05:51 - INFO - __main__ - Instantaneous batch size per device = 1
02/05/2024 20:05:51 - INFO - __main__ - Total train batch size (w. parallel, distributed & accumulation) = 4
02/05/2024 20:05:51 - INFO - __main__ - Gradient Accumulation steps = 4
02/05/2024 20:05:51 - INFO - __main__ - Total optimization steps = 12500
Steps: 0%| | 0/12500 [00:00
main(args)
File "/mnt/sdb/数据2/diffusers/examples/controlnet/train_controlnet.py", line 1007, in main
for step, batch in enumerate(train_dataloader):
File "/home/ps/anaconda3/envs/diffusers/lib/python3.10/site-packages/accelerate/data_loader.py", line 384, in __iter__
current_batch = next(dataloader_iter)
File "/home/ps/anaconda3/envs/diffusers/lib/python3.10/site-packages/torch/utils/data/dataloader.py", line 633, in __next__
data = self._next_data()
File "/home/ps/anaconda3/envs/diffusers/lib/python3.10/site-packages/torch/utils/data/dataloader.py", line 677, in _next_data
data = self._dataset_fetcher.fetch(index) # may raise StopIteration
File "/home/ps/anaconda3/envs/diffusers/lib/python3.10/site-packages/torch/utils/data/_utils/fetch.py", line 49, in fetch
data = self.dataset.__getitems__(possibly_batched_index)
File "/home/ps/anaconda3/envs/diffusers/lib/python3.10/site-packages/datasets/arrow_dataset.py", line 2799, in __getitems__
batch = self.__getitem__(keys)
File "/home/ps/anaconda3/envs/diffusers/lib/python3.10/site-packages/datasets/arrow_dataset.py", line 2795, in __getitem__
return self._getitem(key)
File "/home/ps/anaconda3/envs/diffusers/lib/python3.10/site-packages/datasets/arrow_dataset.py", line 2780, in _getitem
formatted_output = format_table(
File "/home/ps/anaconda3/envs/diffusers/lib/python3.10/site-packages/datasets/formatting/formatting.py", line 629, in format_table
return formatter(pa_table, query_type=query_type)
File "/home/ps/anaconda3/envs/diffusers/lib/python3.10/site-packages/datasets/formatting/formatting.py", line 400, in __call__
return self.format_batch(pa_table)
File "/home/ps/anaconda3/envs/diffusers/lib/python3.10/site-packages/datasets/formatting/formatting.py", line 515, in format_batch
return self.transform(batch)
File "/mnt/sdb/diffusers/examples/controlnet/train_controlnet.py", line 681, in preprocess_train
images = [image.convert("RGB") for image in examples[image_column]]
File "/mnt/sdb/diffusers/examples/controlnet/train_controlnet.py", line 681, in
images = [image.convert("RGB") for image in examples[image_column]]
AttributeError: 'str' object has no attribute 'convert'
Steps: 0%| | 0/12500 [00:00
sys.exit(main())
File "/home/ps/anaconda3/envs/diffusers/lib/python3.10/site-packages/accelerate/commands/accelerate_cli.py", line 47, in main
args.func(args)
File "/home/ps/anaconda3/envs/diffusers/lib/python3.10/site-packages/accelerate/commands/launch.py", line 986, in launch_command
simple_launcher(args)
File "/home/ps/anaconda3/envs/diffusers/lib/python3.10/site-packages/accelerate/commands/launch.py", line 628, in simple_launcher
raise subprocess.CalledProcessError(returncode=process.returncode, cmd=cmd)
subprocess.CalledProcessError: Command '['/home/ps/anaconda3/envs/diffusers/bin/python3.10', 'train_controlnet.py', '--pretrained_model_name_or_path=/home/ps/train_sd_code/diffusers/runwayml/stable-diffusion-v1-5', '--output_dir=./output_test1', '--dataset_name=fusing/fill50k', '--resolution=512', '--learning_rate=1e-5', '--validation_image', './conditioning_image_1.png', './conditioning_image_2.png', '--validation_prompt', 'red circle with blue background', 'cyan circle with brown floral background', '--train_batch_size=1', '--gradient_accumulation_steps=4']' returned non-zero exit status 1.
```

### System Info

I tried diffusers-0.25.0.dev0 and diffusers-0.26.0.dev0, but both have the same problem

### Who can help?

@sayakpaul @yiyixuxu @DN6 @patrickvonplaten

Empieza en examples/controlnet/train_controlnet.py, en preprocess_train, especialmente en la línea 681, y compara las columnas y los valores del dataset fill50k con el comando del README. Reproduce el fallo con el comando accelerate launch proporcionado e inspecciona cómo se formatea el dataset antes del preprocesamiento. Se considera terminado cuando el entrenamiento puede comenzar sin AttributeError.

Escrito por el modelo de indexación a partir del texto del issue.