[Bug] labels of glue/ax are all -1 
This is the test set given by the Glue benchmark. The labels are not provided, and therefore set to -1. Ah, yeah. Why it didn’t occur to me. 😂
Thank you for your comment.
```
ax = nlp.load_dataset('glue', 'ax')
for i in range(30): print(ax['test'][i]['label'], end=', ')
```
```
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 
```