[s2s] distributed eval cleanup
Fixes #7176 

- [x] Fix determinism issue caused by hardcoding num_beams
- [x] local_rank 0 logging
- [x] local_rank 0 tqdm
- [x] same results as `run_eval.py`
- [x] deeper investigation of non-determinism. Gens the same. Labels different?
- [x] save json to one line.

cc @stas00 