Error in metric.compute: missing `original_instructions` argument
I'm running into an error using metrics for computation in the latest master as well as version 0.2.1. Here is a minimal example:

```python
import nlp
rte_metric = nlp.load_metric('glue', name="rte")
rte_metric.compute(
    [0, 0, 1, 1],
    [0, 1, 0, 1],
)
```

```
    181             # Read the predictions and references
    182             reader = ArrowReader(path=self.data_dir, info=None)
--> 183             self.data = reader.read_files(node_files)
    184 
    185             # Release all of our locks

TypeError: read_files() missing 1 required positional argument: 'original_instructions'
```

I believe this might have been introduced with cc8d2508b75f7ba0e5438d0686ee02dcec43c7f4, which added the `original_instructions` argument. Elsewhere, an empty-string default is provided--perhaps that could be done here too?