seq2seq: fail gracefully when predicting using --device cpu and --fp16
When using the recommended seq2seq evaluation procedure in https://github.com/huggingface/transformers/blob/master/examples/seq2seq/README.md :

```
export DATA_DIR=wmt_en_ro
./run_eval.py t5-base \
    $DATA_DIR/val.source t5_val_generations.txt \
    --reference_path $DATA_DIR/val.target \
    --score_path enro_bleu.json \
    --task translation_en_to_ro \
    --n_obs 100 \
    --device cuda \
    --fp16 \
    --bs 32
```

If `--device cuda` is switched to `--device cpu` then it eventually fails with a pytorch error if `--fp16` is also enabled:

```
"threshold_cpu" not implemented for 'Half'
```

It seems that fp16 and cpu evaluation may currently be incompatible.
