Load_dataset for local CSV files
The method load_dataset fails to correctly load a dataset from csv. 

Moreover, I am working on a token-classification task ( POS tagging) , where each row in my CSV contains two columns each of them having a list of strings.
row example:
```tokens  |  labels
['I' , 'am', 'John']  |  ['PRON', 'AUX', 'PROPN' ] 
```
The method, loads each list as a string:  (i.g "['I' , 'am', 'John']").
To solve this issue, I copied the Datasets.Features, created Sequence types ( instead of Value)  and tried to cast the features type
```
new_features['tokens'] = Sequence(feature=Value(dtype='string', id=None))
new_features['labels'] = Sequence(feature=ClassLabel(num_classes=len(tag2idx), names=list(unique_tags)))
dataset = dataset.cast(new_features)
```
but I got the following error 
```
ArrowNotImplementedError: Unsupported cast from string to list using function cast_list
```
Moreover, I tried to set feature parameter in load_dataset method, to my new_features, but this fails as well.
How can this be solved ?
Hi,

this is not a standard CSV file (requires additional preprocessing) so I wouldn't label this as s bug. You could parse the examples with the regex module or the string API to extract the data, but the following approach is probably the easiest (once you load the data):
```python
import ast
# load the dataset and copy the features
def process(ex):
    return {"tokens": ast.literal_eval(ex["tokens"]), "labels": ast.literal_eval(ex["labels"])}
dataset = dataset.map(process, features=new_features)
```
 Hi,

Thanks for the reply.
I  have already used  ```ast.literal_eval``` to evaluate the string into list, but I was getting another error:
```
ArrowInvalid: Could not convert X with type str: tried to convert to int
```
Why this happens ? Should labels be mapped to their ids and use int instead of str ? Yes, just map the labels to their ids.