[s2s] run_eval supports --prefix clarg.
- Useful for multilingual models. 
- `model.config.prefix` will still be used if prefix not passed. This prefix is added to the beginning of each example from the  source document before calling `generate`. 
- `decoder_start_token_id` is a different thing and unaffected.


Usage:

```bash
export dd=wmt_en_de
python run_eval.py Helsinki-NLP/opus-mt-en-gem \
	$dd/val.source \
	$dd/marian_multi_val_gens.txt \
	--reference_path $dd/val.target \
	--task translation --fp16  --bs 128  \
	--score_path $dd/marian_multi_val_bleu.json \
	--prefix ">>deu<<"
```