[s2s] run_eval.py parses generate_kwargs
You can now run

```bash
td=test_data/wmt_en_ro
python run_eval.py t5-base $td/val.source preds.txt --reference_path $td/val.target \
        --score_path metrics.json --length_penalty 0.2 \
         --task translation_en_to_ro --num_beams 2 --n_obs 2 --bs 1 --length_penalty 0.6
```

h/t @stas00 in #6369 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6948?src=pr&el=h1) Report
> Merging [#6948](https://codecov.io/gh/huggingface/transformers/pull/6948?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/6078b12098337bcb98c0540b07a623223ffdd1c8?el=desc) will **increase** coverage by `0.52%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6948/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6948?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6948      +/-   ##
==========================================
+ Coverage   80.01%   80.54%   +0.52%     
==========================================
  Files         161      161              
  Lines       30120    30120              
==========================================
+ Hits        24102    24259     +157     
+ Misses       6018     5861     -157     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6948?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6948/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `24.53% <0.00%> (-63.81%)` | :arrow_down: |
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/6948/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `86.63% <0.00%> (-5.27%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6948/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `84.21% <0.00%> (-2.26%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6948/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.29% <0.00%> (+0.32%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/6948/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `98.95% <0.00%> (+73.82%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6948?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6948?src=pr&el=footer). Last update [6078b12...2f212d3](https://codecov.io/gh/huggingface/transformers/pull/6948?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Yes, please and thank you!!! Needing it badly!<|||||>Here you go!