Better error msg if `len(predictions)` doesn't match `len(references)` in metrics
Can't test this now but it may be a good improvement indeed. I added a function, but it only works with the `list` type. For arrays/tensors, we delegate formatting to the frameworks. 
Improve the error message in `Metric.add_batch` if `len(predictions)` doesn't match `len(references)`.

cc: @BramVanroy (feel free to test this code on your examples and review this PR)