Add test coverage for run_distributed_eval.py that can run on 1 GPU.
The command:

python -m torch.distributed.launch --nproc_per_node=1 run_distributed_eval.py --model_name Helsinki-NLP/opus-mt-en-ro --save_dir opus_wmt_en_ro_gens --data_dir wmt_en_ro

works on 1 GPU.

After adding test coverage, we could try to improve API consistency between run_distributed_eval.py and run_eval.py .