[examples s2s] run_seq2seq.py tweaks
Would it be possible to sync the new `run_seq2seq.py` with `./finetune_trainer.py` outputs?

Before:
```
2021-02-05 12:59:55 | INFO | __main__ | ***** train metrics *****
2021-02-05 12:59:55 | INFO | __main__ |   epoch = 1.0
2021-02-05 12:59:55 | INFO | __main__ |   train_n_objs = 60
2021-02-05 12:59:55 | INFO | __main__ |   train_runtime = 9.7768
2021-02-05 12:59:55 | INFO | __main__ |   train_samples_per_second = 6.137
2021-02-05 12:59:55 | INFO | __main__ | *** Evaluate ***
[INFO|trainer.py:1600] 2021-02-05 12:59:55,434 >> ***** Running Evaluation *****
[INFO|trainer.py:1601] 2021-02-05 12:59:55,434 >>   Num examples = 10
[INFO|trainer.py:1602] 2021-02-05 12:59:55,434 >>   Batch size = 1

2021-02-05 13:00:00 | INFO | __main__ | ***** val metrics *****
2021-02-05 13:00:00 | INFO | __main__ |   epoch = 1.0
2021-02-05 13:00:00 | INFO | __main__ |   val_bleu = 33.3125
2021-02-05 13:00:00 | INFO | __main__ |   val_gen_len = 50.1
2021-02-05 13:00:00 | INFO | __main__ |   val_loss = inf
2021-02-05 13:00:00 | INFO | __main__ |   val_n_objs = 10
2021-02-05 13:00:00 | INFO | __main__ |   val_runtime = 4.7266
2021-02-05 13:00:00 | INFO | __main__ |   val_samples_per_second = 2.116
2021-02-05 13:00:00 | INFO | __main__ | *** Predict ***

```
With the new script:
```
02/05/2021 13:00:41 - INFO - __main__ -   ***** Train results *****
02/05/2021 13:00:41 - INFO - __main__ -     epoch = 1.0
02/05/2021 13:00:41 - INFO - __main__ -     train_runtime = 1.33
02/05/2021 13:00:41 - INFO - __main__ -     train_samples_per_second = 3.008
02/05/2021 13:00:41 - INFO - __main__ -   *** Evaluate ***
***** Running Evaluation *****
  Num examples = 100
  Batch size = 32
02/05/2021 13:00:42 - INFO - __main__ -   ***** Eval results *****
02/05/2021 13:00:42 - INFO - __main__ -     epoch = 1.0
02/05/2021 13:00:42 - INFO - __main__ -     eval_bleu = 1.3059269919149237
02/05/2021 13:00:42 - INFO - __main__ -     eval_gen_len = 17.66
02/05/2021 13:00:42 - INFO - __main__ -     eval_loss = 5.084951400756836
02/05/2021 13:00:42 - INFO - __main__ -     eval_runtime = 0.7079
02/05/2021 13:00:42 - INFO - __main__ -     eval_samples_per_second = 141.261
```

As you can see:
1. the metrics numbers aren't rounded up
2. missing `*_n_obj` in metrics
3. logging is inconsistent.

   the old script has its own issues with log consistency, this one introduces its own inconsistencies.

   To make it easy to read the results - the user-targeted output should ideally be aligned on one side and not in 2 columns - i.e. left column date, etc. right column information.  In the new script `***** Running Evaluation *****` is missing the logger prefix.

Thank you!

@patil-suraj, @sgugger 
this has been resolved in various PRs. It's looking even better than before.