Smarter prediction loop and no- -> no_ in console args
# What does this PR do?

This PR does two things:
- the first one is to replace `no-` to `no_` in the `HFArgumentParser` so that arguments get a more consistent name: for instance `use_tokenizer_fast` in the new examples script give an argument `no-use_tokenizer_fast` and the inconsistency between - and _ makes it hard to find.
- the second one is to avoid computing the predictions and labels (and storing them) in the evaluation of a `Trainer` when there is no `compute_metrics` function.