Fix metric compute (original_instructions missing)
Awesome! This is working now:

```python
import nlp 
seqeval = nlp.load_metric("seqeval") 
y_true = [['O', 'O', 'O', 'B-MISC', 'I-MISC', 'I-MISC', 'O'], ['B-PER', 'I-PER', 'O']] 
y_pred = [['O', 'O', 'B-MISC', 'I-MISC', 'I-MISC', 'I-MISC', 'O'], ['B-PER', 'I-PER', 'O']]                                                                                                                                                                                    

results = seqeval.compute(y_true, y_pred)
```

I heavily need this fix for an upcoming `nlp` integration PR for Transformers (token classification example)  😅 Haha nice ! We'll ship this fix with the next release that will probably come out on thursday :)
When loading arrow data we added in cc8d250 a way to specify the instructions that were used to store them with the loaded dataset.
However metrics load data the same way but don't need instructions (we use one single file).

In this PR I just make `original_instructions` optional when reading files to load a `Dataset` object.

This should fix #269 