IterableDataset: Unsupported ScalarType BFloat16
### Describe the bug

`IterableDataset.from_generator` crashes when using BFloat16:

```
  File "/usr/local/lib/python3.11/site-packages/datasets/utils/_dill.py", line 169, in _save_torchTensor
    args = (obj.detach().cpu().numpy(),)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^

TypeError: Got unsupported ScalarType BFloat16
```

### Steps to reproduce the bug

```python
import torch
from datasets import IterableDataset


def demo(x):
    yield {"x": x}


x = torch.tensor([1.], dtype=torch.bfloat16)

dataset = IterableDataset.from_generator(
    demo,
    gen_kwargs=dict(x=x),
)

example = next(iter(dataset))
print(example)
```

### Expected behavior

Code sample should print:
```python
{'x': tensor([1.], dtype=torch.bfloat16)}
```

### Environment info

```
datasets==2.20.0
torch==2.2.2
```