[wip] [pegasus] encode s/\r?\n/<n>/g + test
As discussed in https://github.com/huggingface/transformers/issues/7743

* [x] On encode: `s/\r?\n/<n>/g` + test
* [ ] On decode: `s/<n>/\n/g` - not sure what to do since `run_eval.py` can't work with multiline outputs as it uses readline to get each record and inserting `\n` would break the records if they are now multiline.

@sshleifer 

Fixes: #7743
I have some concerns defaulting this logic to `True`, given that some `sshleifer/` models like `sshleifer/pegasus-cnn-ft-v2/`, `distill-pegasus-cnn-16-4/` were trained without newlines in the dataset.  I'm sorry I forgot to mention these until now.

I would merge this if the logic to do the regex were defaulted to False and we checked that `PegasusFastTokenizer` did the same thing.
I would also be fine abandoning, since you have already solved the Pegasus replication issue with your `build/` scripts, and we haven't had anybody external ask for this yet.<|||||>Sure, we can just leave it unmerged and if someone reports an issue get back to sorting this out. It was a trivial change, so I have no problem with closing this. It's your call, @sshleifer.<|||||>will reopen if needed, thx for being flexible @stas00 