wikiann dataset is missing columns 
Cool ! Let me give you some context:

#### Contribution guide

You can find the contribution guide here:

https://github.com/huggingface/datasets/blob/master/CONTRIBUTING.md

It explains how to set up your dev environment in a few steps.

#### Dataset loading

Each Dataset is defined by a Table that have many rows (one row = one example) and columns (one column = one feature).
To change how a dataset is constructed, you have to modify its dataset script that you can find here:

https://github.com/huggingface/datasets/blob/master/datasets/wikiann/wikiann.py

It includes everything needed to load the WikiANN dataset.
You can load locally a modified version of `wikiann.py` with `load_dataset("path/to/wikiann.py")`.

#### Define a new column

Each column has a name and a type. You can see how the features of WikiANN are defined here:

https://github.com/huggingface/datasets/blob/c98e4b8f23e3770c401c6d9326e243e1ffd599ec/datasets/wikiann/wikiann.py#L245-L263

Ideally we would have one additional feature "spans":
```python
        "spans": datasets.Sequence(datasets.Value("string")),
```

#### Compute the content of each row

To build the WikiANN rows, the _generate_examples method from [here](https://github.com/huggingface/nlp/blob/c98e4b8f23e3770c401c6d9326e243e1ffd599ec/datasets/wikiann/wikiann.py#L292-L316) is used. This function `yield` one python dictionary for each example:
```python
yield guid_index, {"tokens": tokens, "ner_tags": ner_tags, "langs": langs}
```

The objective would be to return instead something like
```python
spans = spans = get_spans(tokens, tags)
yield guid_index, {"tokens": tokens, "ner_tags": ner_tags, "langs": langs, "spans": spans}
```

Let me know if you have questions !
Hi
Wikiann dataset needs to have "spans" columns, which is necessary to be able to use this dataset, but this column is missing from huggingface datasets, could you please have a look? thank you @lhoestq 
wikiann dataset is missing columns  
 Hi
Wikiann dataset needs to have "spans" columns, which is necessary to be able to use this dataset, but this column is missing from huggingface datasets, could you please have a look? thank you @lhoestq  
 Cool ! Let me give you some context:

#### Contribution guide

You can find the contribution guide here:

https://github.com/huggingface/datasets/blob/master/CONTRIBUTING.md

It explains how to set up your dev environment in a few steps.

#### Dataset loading

Each Dataset is defined by a Table that have many rows (one row = one example) and columns (one column = one feature).
To change how a dataset is constructed, you have to modify its dataset script that you can find here:

https://github.com/huggingface/datasets/blob/master/datasets/wikiann/wikiann.py

It includes everything needed to load the WikiANN dataset.
You can load locally a modified version of `wikiann.py` with `load_dataset("path/to/wikiann.py")`.

#### Define a new column

Each column has a name and a type. You can see how the features of WikiANN are defined here:

https://github.com/huggingface/datasets/blob/c98e4b8f23e3770c401c6d9326e243e1ffd599ec/datasets/wikiann/wikiann.py#L245-L263

Ideally we would have one additional feature "spans":
```python
        "spans": datasets.Sequence(datasets.Value("string")),
```

#### Compute the content of each row

To build the WikiANN rows, the _generate_examples method from [here](https://github.com/huggingface/nlp/blob/c98e4b8f23e3770c401c6d9326e243e1ffd599ec/datasets/wikiann/wikiann.py#L292-L316) is used. This function `yield` one python dictionary for each example:
```python
yield guid_index, {"tokens": tokens, "ner_tags": ner_tags, "langs": langs}
```

The objective would be to return instead something like
```python
spans = spans = get_spans(tokens, tags)
yield guid_index, {"tokens": tokens, "ner_tags": ner_tags, "langs": langs, "spans": spans}
```

Let me know if you have questions !