[s2s] run_eval.py QOL improvements and cleanup

- default to saving metrics to `metrics.json`
- cleanup: delete some dead code.
- save seconds_per_sample, runtime, num_samples

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6746?src=pr&el=h1) Report
> Merging [#6746](https://codecov.io/gh/huggingface/transformers/pull/6746?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a75c64d80c76c3dc71f735d9197a4a601847e0cd?el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6746/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6746?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6746      +/-   ##
==========================================
- Coverage   78.96%   78.93%   -0.04%     
==========================================
  Files         157      157              
  Lines       28486    28486              
==========================================
- Hits        22495    22485      -10     
- Misses       5991     6001      +10     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6746?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6746/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `80.70% <0.00%> (-2.26%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6746/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.41% <0.00%> (-0.26%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6746?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6746?src=pr&el=footer). Last update [a75c64d...0b9f1cd](https://codecov.io/gh/huggingface/transformers/pull/6746?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
