[examples/s2s] add test set predictions
# What does this PR do?

This PR adds the `do_predict` option to the `run_seq2seq.py` script for test set predictions.

Fixes #10032

cc. @stas00
> I propose that the best approach would be to finish everything that is planned and then we will run tests side by side and note any small discrepancies if any and fix them in one go? Does that work?

Yes, this was the last major missing piece from this script. Now I'm going to start running both scripts side by side (manually converting the old datasets to new datasets format) and note the discrepancies, I'll also wait for your tests.

> I'm waiting for the datasets hub to port the datasets to be able to compare the old and the new.

Let's not wait for the hub, for now, we could just manually convert the datasets for tests and later upload them to the hub once it's ready. <|||||>> > I propose that the best approach would be to finish everything that is planned and then we will run tests side by side and note any small discrepancies if any and fix them in one go? Does that work?
> 
> Yes, this was the last major missing piece from this script. Now I'm going to start running both scripts side by side (manually converting the old datasets to new datasets format) and note the discrepancies, I'll also wait for your tests.

That works.

> > I'm waiting for the datasets hub to port the datasets to be able to compare the old and the new.
> 
> Let's not wait for the hub, for now, we could just manually convert the datasets for tests and later upload them to the hub once it's ready.

Sure - I already wrote the code for wmt en-ro https://github.com/huggingface/transformers/issues/10044#issuecomment-774413928 need to adapt to others.<|||||>Changed `eval_beams` to `num_beams`. Hopefully final name change. Merging!