Unable to use dataset with PyTorch dataloader
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Hi! `para_crawl` has a single column of type `Translation`, which stores translation dictionaries. These dictionaries can be stored in a NumPy array but not in a PyTorch tensor since PyTorch only supports numeric types. In `datasets`, the conversion to `torch` works as follows: 
1. convert PyArrow table to NumPy arrays 
2. convert NumPy arrays to Torch tensors. 

The 2nd step is problematic for your case as `datasets` attempts to convert the array of dictionaries to a PyTorch tensor. One way to fix this is to use the [preprocessing logic](https://github.com/huggingface/transformers/blob/8581a798c0a48fca07b29ce2ca2ef55adcae8c7e/examples/pytorch/translation/run_translation.py#L440-L458) from the Transformers translation script. And on our side, I think we can replace a NumPy array of dicts with a dict of NumPy array if the feature type is `Translation`/`TranslationVariableLanguages` (one array for each language) to get the official PyTorch error message for strings in such case.
## 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
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `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

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