Issue: List of images behave differently on IterableDataset and Dataset

### Describe the bug

This code:

```python
def train_iterable_gen():
        images = np.array(load_image("https://cdn.britannica.com/61/93061-050-99147DCE/Statue-of-Liberty-Island-New-York-Bay.jpg").resize((128, 128)))
        yield {
            "images": np.expand_dims(images, axis=0),
            "messages": [
                {
                    "role": "user",
                    "content": [{"type": "image", "url": "https://cdn.britannica.com/61/93061-050-99147DCE/Statue-of-Liberty-Island-New-York-Bay.jpg" }]
                },
                {
                    "role": "assistant",
                    "content": [{"type": "text", "text": "duck" }]
                }
            ]
        }
    train_ds = Dataset.from_generator(train_iterable_gen,
                                      features=Features({
                                           'images': [datasets.Image(mode=None, decode=True, id=None)],
                                           'messages': [{'content': [{'text': datasets.Value(dtype='string', id=None), 'type': datasets.Value(dtype='string', id=None) }], 'role': datasets.Value(dtype='string', id=None)}]
                                           } )
                                           )
```

works as I'd expect; if I iterate the dataset then the `images` column returns a `List[PIL.Image.Image]`, i.e. `'images': [<PIL.PngImagePlugin.PngImageFile image mode=RGB size=128x128 at 0x77EFB7EF4680>]`.

But if I change `Dataset` to `IterableDataset`, the `images` column changes into `'images': [{'path': None, 'bytes': ..]`

### Steps to reproduce the bug

The code above +

```python
def load_image(url):
        response = requests.get(url)
        image = Image.open(io.BytesIO(response.content))
        return image
```

I'm feeding it to SFTTrainer

### Expected behavior

Dataset and IterableDataset would behave the same

### Environment info

```yaml
requires-python = ">=3.12"
dependencies = [
    "av>=14.1.0",
    "boto3>=1.36.7",
    "datasets>=3.3.2",
    "docker>=7.1.0",
    "google-cloud-storage>=2.19.0",
    "grpcio>=1.70.0",
    "grpcio-tools>=1.70.0",
    "moviepy>=2.1.2",
    "open-clip-torch>=2.31.0",
    "opencv-python>=4.11.0.86; sys_platform == 'darwin'",
    "opencv-python-headless>=4.11.0.86; sys_platform == 'linux'",
    "pandas>=2.2.3",
    "pillow>=10.4.0",
    "plotly>=6.0.0",
    "py-spy>=0.4.0",
    "pydantic>=2.10.6",
    "pydantic-settings>=2.7.1",
    "pymysql>=1.1.1",
    "ray[data,default,serve,train,tune]>=2.43.0",
    "torch>=2.6.0",
    "torchmetrics>=1.6.1",
    "torchvision>=0.21.0",
    "transformers[torch]@git+https://github.com/huggingface/transformers",
    "wandb>=0.19.4",
    # https://github.com/Dao-AILab/flash-attention/issues/833
    "flash-attn @ https://github.com/Dao-AILab/flash-attention/releases/download/v2.7.3/flash_attn-2.7.3+cu12torch2.6cxx11abiFALSE-cp312-cp312-linux_x86_64.whl; sys_platform == 'linux'",
    "trl@https://github.com/huggingface/trl.git",
    "peft>=0.14.0",
]
```

Comments:
Hi ! Can you try with `datasets` ^3.4 released recently ? on my side it works with IterableDataset on the recent version :)

```python
In [20]: def train_iterable_gen():
    ...:         images = np.array(load_image("https://cdn.britannica.com/61/93061-050-99147DCE/Statue-of-Liberty-Island-New-York-Bay.jpg").resize((128, 128)))
    ...:         yield {
    ...:             "images": np.expand_dims(images, axis=0),
    ...:             "messages": [
    ...:                 {
    ...:                     "role": "user",
    ...:                     "content": [{"type": "image", "url": "https://cdn.britannica.com/61/93061-050-99147DCE/Statue-of-Liberty-Island-New-York-Bay.jpg" }]
    ...:                 },
    ...:                 {
    ...:                     "role": "assistant",
    ...:                     "content": [{"type": "text", "text": "duck" }]
    ...:                 }
    ...:             ]
    ...:         }
    ...: 
    ...: train_ds = IterableDataset.from_generator(train_iterable_gen,
    ...:                                     features=Features({
    ...:                                         'images': [datasets.Image(mode=None, decode=True, id=None)],
    ...:                                         'messages': [{'content': [{'text': datasets.Value(dtype='string', id=None), 'type': datasets.Value(dtype='string', id=None) }],
    ...: 'role': datasets.Value(dtype='string', id=None)}]
    ...:                                         } )
    ...:                                         )


In [21]: 

In [21]: next(iter(train_ds))
/Users/quentinlhoest/hf/datasets/src/datasets/features/image.py:338: UserWarning: Downcasting array dtype int64 to uint8 to be compatible with 'Pillow'
  warnings.warn(f"Downcasting array dtype {dtype} to {dest_dtype} to be compatible with 'Pillow'")
Out[21]: 
{'images': [<PIL.PngImagePlugin.PngImageFile image mode=RGB size=128x128>],
 'messages': [{'content': [{'text': None, 'type': 'image'}], 'role': 'user'},
  {'content': [{'type': 'text', 'text': 'duck'}], 'role': 'assistant'}]}
```

Hm I tried it here and it works as expected, even on datasets 3.3.2. I guess maybe something in the SFTTrainer is doing additional processing on the dataset, I'll have a look there.

Thanks @lhoestq!