Issue: ImageFolder with Grayscale images dataset

Hi, I'm facing a problem with a grayscale images dataset I have uploaded [here](https://huggingface.co/datasets/ChainYo/rvl-cdip) (RVL-CDIP)

I'm getting an error while I want to use images for training a model with PyTorch DataLoader. Here is the full traceback:

```bash
AttributeError: Caught AttributeError in DataLoader worker process 0.
Original Traceback (most recent call last):
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/torch/utils/data/_utils/worker.py", line 287, in _worker_loop
    data = fetcher.fetch(index)
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/torch/utils/data/_utils/fetch.py", line 49, in fetch
    data = [self.dataset[idx] for idx in possibly_batched_index]
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/torch/utils/data/_utils/fetch.py", line 49, in <listcomp>
    data = [self.dataset[idx] for idx in possibly_batched_index]
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/datasets/arrow_dataset.py", line 1765, in __getitem__
    return self._getitem(
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/datasets/arrow_dataset.py", line 1750, in _getitem
    formatted_output = format_table(
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/datasets/formatting/formatting.py", line 532, in format_table
    return formatter(pa_table, query_type=query_type)
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/datasets/formatting/formatting.py", line 281, in __call__
    return self.format_row(pa_table)
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/datasets/formatting/torch_formatter.py", line 58, in format_row
    return self.recursive_tensorize(row)
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/datasets/formatting/torch_formatter.py", line 54, in recursive_tensorize
    return map_nested(self._recursive_tensorize, data_struct, map_list=False)
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/datasets/utils/py_utils.py", line 314, in map_nested
    mapped = [
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/datasets/utils/py_utils.py", line 315, in <listcomp>
    _single_map_nested((function, obj, types, None, True, None))
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/datasets/utils/py_utils.py", line 267, in _single_map_nested
    return {k: _single_map_nested((function, v, types, None, True, None)) for k, v in pbar}
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/datasets/utils/py_utils.py", line 267, in <dictcomp>
    return {k: _single_map_nested((function, v, types, None, True, None)) for k, v in pbar}
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/datasets/utils/py_utils.py", line 251, in _single_map_nested
    return function(data_struct)
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/datasets/formatting/torch_formatter.py", line 51, in _recursive_tensorize
    return self._tensorize(data_struct)
  File "/home/chainyo/miniconda3/envs/gan-bird/lib/python3.8/site-packages/datasets/formatting/torch_formatter.py", line 38, in _tensorize
    if np.issubdtype(value.dtype, np.integer):
AttributeError: 'bytes' object has no attribute 'dtype'
```

I don't really understand why the image is still a bytes object while I used transformations on it. Here the code I used to upload the dataset (and it worked well):

```python
train_dataset = load_dataset("imagefolder", data_dir="data/train")
train_dataset = train_dataset["train"]
test_dataset = load_dataset("imagefolder", data_dir="data/test")
test_dataset = test_dataset["train"]
val_dataset = load_dataset("imagefolder", data_dir="data/val")
val_dataset = val_dataset["train"]

dataset = DatasetDict({
    "train": train_dataset,
    "val": val_dataset,
    "test": test_dataset
})
dataset.push_to_hub("ChainYo/rvl-cdip")
```

Now here is the code I am using to get the dataset and prepare it for training:

```python
img_size = 512
batch_size = 128
normalize = [(0.5), (0.5)]
data_dir = "ChainYo/rvl-cdip"

dataset = load_dataset(data_dir, split="train")

transforms = transforms.Compose([
        transforms.Resize(img_size), 
        transforms.CenterCrop(img_size), 
        transforms.ToTensor(), 
        transforms.Normalize(*normalize)
])

transformed_dataset = dataset.with_transform(transforms)
transformed_dataset.set_format(type="torch", device="cuda")

train_dataloader = torch.utils.data.DataLoader(
    transformed_dataset, batch_size=batch_size, shuffle=True, num_workers=4, pin_memory=True
)
```

But this get me the error above. I don't understand why it's doing this kind of weird thing?
Do I need to map something on the dataset? Something like this:

```python
labels = dataset.features["label"].names
num_labels = dataset.features["label"].num_classes


def preprocess_data(examples):
    images = [ex.convert("RGB") for ex in examples["image"]]
    labels = [ex for ex in examples["label"]]
    return {"images": images, "labels": labels}


features = Features({
    "images": Image(decode=True, id=None),
    "labels": ClassLabel(num_classes=num_labels, names=labels)
})


decoded_dataset = dataset.map(preprocess_data, remove_columns=dataset.column_names, features=features, batched=True, batch_size=100)
```



Comments:
Hi! Replacing:
```python
transformed_dataset = dataset.with_transform(transforms)
transformed_dataset.set_format(type="torch", device="cuda")
```

with:
```python
def transform_func(examples):
    examples["image"] = [transforms(img).to("cuda") for img in examples["image"]]
    return examples

transformed_dataset = dataset.with_transform(transform_func)
```
should fix the issue. `datasets` doesn't support chaining of transforms (you can think of `set_format`/`with_format` as a predefined transform func for `set_transform`/`with_transforms`), so the last transform (in your case, `set_format`) takes precedence over the previous ones (in your case `with_format`). And the PyTorch formatter is not supported by the Image feature, hence the error (adding support for that is on our short-term roadmap).

Ok thanks a lot for the code snippet!

I love the way `datasets` is easy to use but it made it really long to pre-process all the images (400.000 in my case) before training anything. `ImageFolder` from pytorch is faster in my case but force me to have the images on my local machine.

I don't know how to speed up the process without switching to `ImageFolder` :smile: 

You can pass `ignore_verifications=True` in `load_dataset` to skip checksum verification, which takes a lot of time if the number of files is large. We will consider making this the default behavior.