Issue: Error iteration over IterableDataset using Torch DataLoader

## Describe the bug
I have an IterableDataset (created using streaming=True) and I am trying to create batches using Torch DataLoader class by passing this IterableDataset to it. This throws error which is pasted below. I can do the same by using Torch IterableDataset. One thing I noticed is that in the former case when I look at the dataloader.sampler class I get torch.utils.data.sampler.SequentialSampler while the latter one gives torch.utils.data.dataloader._InfiniteConstantSampler. 

I am not sure if this is how it is meant to be used, but that's what seemed reasonable to me. 

## Steps to reproduce the bug

1. Does not work.
```python
>>> from datasets import load_dataset
>>> dataset = load_dataset('oscar', "unshuffled_deduplicated_en", split='train', streaming=True)
>>> dataloader = torch.utils.data.DataLoader(dataset, batch_size=4)
>>> dataloader.sampler
<torch.utils.data.sampler.SequentialSampler object at 0x7f245a510208>
>>> for batch in dataloader:
...     print(batch)
```

2. Works.
```python
import torch
from torch.utils.data import Dataset, IterableDataset, DataLoader
class CustomIterableDataset(IterableDataset):
  'Characterizes a dataset for PyTorch'
  def __init__(self, data):
        'Initialization'
        self.data = data


  def __iter__(self):
        return iter(self.data)


data = list(range(12))
dataset = CustomIterableDataset(data)
dataloader = DataLoader(dataset, batch_size=4)
print("dataloader: ", dataloader.sampler)
for batch in dataloader:
    print(batch)
```

## Expected results
To get batches of data with the batch size as 4. Output from the latter one (2) though Datasource is different here so actual data is different.
dataloader:  <torch.utils.data.dataloader._InfiniteConstantSampler object at 0x7f1cc29e2c50>
tensor([0, 1, 2, 3])
tensor([4, 5, 6, 7])
tensor([ 8,  9, 10, 11])

## Actual results
<torch.utils.data.sampler.SequentialSampler object at 0x7f245a510208>

...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/data/leshekha/lib/HFDatasets/lib/python3.6/site-packages/torch/utils/data/dataloader.py", line 435, in __next__
    data = self._next_data()
  File "/data/leshekha/lib/HFDatasets/lib/python3.6/site-packages/torch/utils/data/dataloader.py", line 474, in _next_data
    index = self._next_index()  # may raise StopIteration
  File "/data/leshekha/lib/HFDatasets/lib/python3.6/site-packages/torch/utils/data/dataloader.py", line 427, in _next_index
    return next(self._sampler_iter)  # may raise StopIteration
  File "/data/leshekha/lib/HFDatasets/lib/python3.6/site-packages/torch/utils/data/sampler.py", line 227, in __iter__
    for idx in self.sampler:
  File "/data/leshekha/lib/HFDatasets/lib/python3.6/site-packages/torch/utils/data/sampler.py", line 67, in __iter__
    return iter(range(len(self.data_source)))
TypeError: object of type 'IterableDataset' has no len()

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: '1.8.1.dev0'
- Platform: Linux
- Python version: Python 3.6.8
- PyArrow version: '3.0.0'


Comments:
Hi ! This is because you first need to format the dataset for pytorch:

```python
>>> import torch
>>> from datasets import load_dataset
>>> dataset = load_dataset('oscar', "unshuffled_deduplicated_en", split='train', streaming=True)
>>> torch_iterable_dataset = dataset.with_format("torch")
>>> assert isinstance(torch_iterable_dataset, torch.utils.data.IterableDataset)
>>> dataloader = torch.utils.data.DataLoader(torch_iterable_dataset, batch_size=4)
>>> next(iter(dataloader))
{'id': tensor([0, 1, 2, 3]), 'text': ['Mtendere Village was inspired...]}
```

This is because the pytorch dataloader expects a subclass of `torch.utils.data.IterableDataset`. Since you can't pass an arbitrary iterable to a pytorch dataloader, you first need to build an object that inherits from `torch.utils.data.IterableDataset` using `with_format("torch")` for example.


Thank you for that and the example! 

What you said makes total sense; I just somehow missed that and assumed HF IterableDataset was a subclass of Torch IterableDataset.