Dataset.unique(column: str) -> ArrowNotImplementedError
Hi @orkenstein, thanks for reporting.

Please note that for this case, our `datasets` library uses under the hood the Apache Arrow `unique` function: https://arrow.apache.org/docs/python/generated/pyarrow.compute.unique.html#pyarrow.compute.unique

And currently the Apache Arrow `unique` function is only implemented for these input types (see info in their [docs](https://arrow.apache.org/docs/cpp/compute.html#array-wise-vector-functions)): Boolean, Null, Numeric, Temporal, Binary- and String-like.

However, the data types of the `wikiann` dataset are all `list<item: string>` (see its [dataset card](https://huggingface.co/datasets/wikiann#data-fields)), and thus, not yet supported by the Apache Arrow `unique` function. As a workaround solution you can use pandas:
```python
from datasets import load_dataset

dataset = load_dataset('wikiann', 'en', split='train')
df = dataset.to_pandas()
unique_df = df[~df.tokens.apply(tuple).duplicated()]  # from https://stackoverflow.com/a/46958336/17517845
```

Note that pandas loads the dataset in memory (this one is small so it's fine). @lhoestq thank you! I will fall back to this method for now
## Describe the bug
I'm trying to use `unique()` function, but it fails
## Steps to reproduce the bug
1. Get dataset
2. Call `unique`
3. Error

# Sample code to reproduce the bug
```python
!pip show datasets

from datasets import load_dataset

dataset = load_dataset('wikiann', 'en')

dataset['train'].column_names
dataset['train'].unique(dataset['train'].column_names[0])
```

## Expected results
It would be nice to actually see unique items

## Actual results
Error:
```python
---------------------------------------------------------------------------
ArrowNotImplementedError                  Traceback (most recent call last)
[<ipython-input-10-5e0de07ed42c>](https://s0qyv2vjaji-496ff2e9c6d22116-0-colab.googleusercontent.com/outputframe.html?vrz=colab-20220324-060046-RC00_436956229#) in <module>()
      6 
      7 dataset['train'].column_names
----> 8 dataset['train'].unique(dataset['train'].column_names[0])

5 frames
/usr/local/lib/python3.7/dist-packages/pyarrow/error.pxi in pyarrow.lib.check_status()

ArrowNotImplementedError: Function unique has no kernel matching input types (array[list<item: string>])
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 2.0.0
- Platform: Google Collab
- Python version: 3.7.13
- PyArrow version: 6.0.1
