bug in SNLI dataset 
Hi ! The labels -1 correspond to the examples without gold labels in the original snli dataset.
Feel free to remove these examples if you don't need them by using
```python
data = data.filter(lambda x: x["label"] != -1)
```
Hi
There is label of -1 in train set of SNLI dataset, please find the code below:

```
import numpy as np 
import datasets 
data = datasets.load_dataset("snli")["train"]
labels = []
for d in data:
   labels.append(d["label"])
print(np.unique(labels))
```

and results:

`[-1  0  1  2]`

version of datasets used:
`datasets                  1.2.1                     <pip>
`

thanks for your help. @lhoestq 