Multidimensional arrays in a Dataset
Hi,

I'm trying to put together a `datasets.Dataset` to be used with LayoutLM which is available in `transformers`. This model requires as input the bounding boxes of each of the token of a sequence. This is when I realized that `Dataset` does not support multi-dimensional arrays as a value for a column in a row.

The following code results in conversion error in pyarrow (`pyarrow.lib.ArrowInvalid: ('Can only convert 1-dimensional array values', 'Conversion failed for column bbox with type object')`)

```
from datasets import Dataset
import pandas as pd
import numpy as np

dataset = pd.DataFrame({
    'bbox': [
        np.array([[1,2,3,4],[1,2,3,4],[1,2,3,4]]),
        np.array([[1,2,3,4],[1,2,3,4],[1,2,3,4]]),
        np.array([[1,2,3,4],[1,2,3,4],[1,2,3,4]]),
        np.array([[1,2,3,4],[1,2,3,4],[1,2,3,4]])
    ],
    'input_ids': [1, 2, 3, 4]
})
dataset = Dataset.from_pandas(dataset)
```

Since I wanted to use pytorch for the downstream training task, I also tried a few ways to directly put in a column of 2-D pytorch tensor in a formatted dataset, but I can only have a list of 1-D tensors, or a list of arrays, or a list of lists.

```
import torch
from datasets import Dataset
import pandas as pd

dataset = pd.DataFrame({
    'bbox': [
        [[1,2,3,4],[1,2,3,4],[1,2,3,4]],
        [[1,2,3,4],[1,2,3,4],[1,2,3,4]],
        [[1,2,3,4],[1,2,3,4],[1,2,3,4]],
        [[1,2,3,4],[1,2,3,4],[1,2,3,4]]
    ],
    'input_ids': [1, 2, 3, 4]
})
dataset = Dataset.from_pandas(dataset)

def test(examples):
    return {'bbbox': torch.Tensor(examples['bbox'])}
dataset = dataset.map(test)
print(dataset[0]['bbox'])
print(dataset[0]['bbbox'])

dataset.set_format(type='torch', columns=['input_ids', 'bbox'], output_all_columns=True)
print(dataset[0]['bbox'])
print(dataset[0]['bbbox'])

def test2(examples):
    return {'bbbox': torch.stack(examples['bbox'])}
dataset = dataset.map(test2)

print(dataset[0]['bbox'])
print(dataset[0]['bbbox'])
```

Is is possible to support n-D arrays/tensors in datasets? 
It seems that it can also be useful for this [feature request](https://github.com/huggingface/datasets/issues/263).