Unable to use dataset with PyTorch dataloader

## Describe the bug

When using `.with_format("torch")`, an arrow table is returned and I am unable to use it by passing it to a PyTorch DataLoader: please see the code below.

## Steps to reproduce the bug
```python
from datasets import load_dataset
from torch.utils.data import DataLoader

ds = load_dataset(
"para_crawl",
name="enfr",
cache_dir="/tmp/test/",
split="train",
keep_in_memory=True,
)

dataloader = DataLoader(ds.with_format("torch"), num_workers=32)
print(next(iter(dataloader)))
```

Is there something I am doing wrong? The documentation does not say much about the behavior of `.with_format()` so I feel like I am a bit stuck here :-/

Thanks in advance for your help!

## Expected results

The code should run with no error

## Actual results

```
AttributeError: 'str' object has no attribute 'dtype'
```

## Environment info

- `datasets` version: 2.3.2
- Platform: Linux-4.18.0-348.el8.x86_64-x86_64-with-glibc2.28
- Python version: 3.10.4
- PyArrow version: 8.0.0
- Pandas version: 1.4.3