How to assign new values to Dataset?
![image](https://user-images.githubusercontent.com/37113676/179149159-bbbda0c8-a661-403c-87ed-dc2b4219cd68.png)

Hi, if I want to change some values of the dataset, or add new columns to it, how can I do it?

For example, I want to change all the labels of the SST2 dataset to `0`:
```python
from datasets import load_dataset
data = load_dataset('glue','sst2')

data['train']['label'] = [0]*len(data)
```

I will get the error:
```
TypeError: 'Dataset' object does not support item assignment
```