[s2s] run_eval/run_eval_search tweaks
working with @sshleifer on making run_eval/run_eval_search and tests better.

sadly I think `pytest.param` was needed or each entry needs to be a list or something:

https://circle-production-customer-artifacts.s3.amazonaws.com/picard/forks/5bdabdd888af1f000130874a/278214696/5f62bdadb33de21283aa7519-0-build/artifacts/test_output.txt?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Date=20200917T014628Z&X-Amz-SignedHeaders=host&X-Amz-Expires=59&X-Amz-Credential=AKIAJR3Q6CR467H7Z55A%2F20200917%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Signature=4ec6c5af819aab91ed8103865ad61af3617a43c8ffa34fe5192a45e3067e3cc0


```
__________ ERROR collecting examples/seq2seq/test_seq2seq_examples.py __________
examples/seq2seq/test_seq2seq_examples.py::test_finetune: in "parametrize" the number of names (1):
  ['model']
must be equal to the number of values (31):
  patrickvonplaten/t5-tiny-random
__________ ERROR collecting examples/seq2seq/test_seq2seq_examples.py __________
```<|||||>> sadly I think `pytest.param` was needed or each entry needs to be a list or something:
right I didn't run all tests, didn't see the other entries used an inefficient use of `parametrize`.

Instead of:
```
@pytest.mark.parametrize("model", [BART_TINY, MBART_TINY])
```
They were using:
```
@pytest.mark.parametrize(["model"], ...
```
notice the argument names are normally a string and not a list. Since it was made into the list, that's why all the extra code.

a recent addition: https://huggingface.co/transformers/master/testing.html#parametrization

BTW, if we switch to `parametrized` (which is now part of the dependencies for `dev`) it works identically with `unittest` and `pytest` tests - same API.

I fixed it and now:

```
pytest --disable-warnings examples/seq2seq/test_seq2seq_examples.py --collect-only -q
[...]
examples/seq2seq/test_seq2seq_examples.py::test_run_eval_slow[sshleifer/bart-tiny-random]
examples/seq2seq/test_seq2seq_examples.py::test_run_eval
examples/seq2seq/test_seq2seq_examples.py::test_run_eval_slow[sshleifer/tiny-mbart]
[...]
```
<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7192?src=pr&el=h1) Report
> Merging [#7192](https://codecov.io/gh/huggingface/transformers/pull/7192?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/45b0b1ff2f28d83c734b7505e1705b799ce2ff84?el=desc) will **increase** coverage by `1.08%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7192/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7192?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7192      +/-   ##
==========================================
+ Coverage   78.41%   79.50%   +1.08%     
==========================================
  Files         169      169              
  Lines       32293    32293              
==========================================
+ Hits        25323    25673     +350     
+ Misses       6970     6620     -350     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7192?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [...c/transformers/modeling\_tf\_transfo\_xl\_utilities.py](https://codecov.io/gh/huggingface/transformers/pull/7192/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90cmFuc2ZvX3hsX3V0aWxpdGllcy5weQ==) | `10.00% <0.00%> (-76.00%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/7192/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG5ldC5weQ==) | `20.85% <0.00%> (-71.41%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/7192/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90cmFuc2ZvX3hsLnB5) | `19.85% <0.00%> (-68.29%)` | :arrow_down: |
| [src/transformers/modeling\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/7192/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tb2JpbGViZXJ0LnB5) | `79.21% <0.00%> (-10.25%)` | :arrow_down: |
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/7192/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `92.83% <0.00%> (-0.72%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7192/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.68% <0.00%> (-0.65%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7192/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.84% <0.00%> (-0.25%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/7192/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `93.91% <0.00%> (-0.14%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/7192/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `88.34% <0.00%> (+63.80%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/7192/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `93.23% <0.00%> (+74.20%)` | :arrow_up: |
| ... and [2 more](https://codecov.io/gh/huggingface/transformers/pull/7192/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7192?src=pr&el=footer). Last update [45b0b1f...d329cf2](https://codecov.io/gh/huggingface/transformers/pull/7192?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
