Converting a Value to a ClassLabel
Hi @nelson-liu!
Here is what I do to convert a string to class label:

```python
from datasets import load_dataset, features


dset = load_dataset(...)
col_name = "the string column name"

class_names  = dset.unique(col_name)
class_feature = features.ClassLabel(names=sorted(class_names))
dset = dset.map(lambda str_value: {col_name: class_feature.str2int(str_value)}, input_columns=col_name)

dset = dset.cast(features.Features({
    ...
    col_name: class_feature
})
```

Hi!

In the docs for `cast`, it's noted that `For non-trivial conversion, e.g. string <-> ClassLabel you should use map() to update the Dataset.`

Would it be possible to have an example  that demonstrates such a string <-> ClassLabel conversion using `map`? Thanks!