fix run_seq2seq.py; porting trainer tests to it
This PR:

- restores some of the essential dropped functionality from `finetune_trainer.py` - I'm almost sure this is far far from complete since so much was just dropped 
- ports wmt_en_ro test data to `jsonlines` - I move the tests dataset into the root of examples so that it can be accessed by a variety of sub-projects.
- ports DeepSpeed tests to use `run_seq2seq.py`
- ports the other trainer script to use `run_seq2seq.py`

@sgugger 
OK, I decided to go ahead and port the other scripts instead of waiting for merging of the first set. Had to make some more fixes in the script while at it.


