[examples] fixes arguments for summarization finetune scripts
This PR fixes arguments in bash scripts for finetuning bart/bart_tiny/t5 models in summarization examples.

After changes that were merged in #4951 i noticed some small problems. That includes:
* missing default arguments for `finetune.py` (--data_dir, output_dir)
* invalid argument name for number of gpu to use (`n_gpu` instead of `gpus`)
* argument `model_type` that is not present in list of arguments and crashes the script
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5157?src=pr&el=h1) Report
> Merging [#5157](https://codecov.io/gh/huggingface/transformers/pull/5157?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/68e19f1c228c92d5d800533f558faff24b57127a&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5157/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5157?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5157   +/-   ##
=======================================
  Coverage   77.93%   77.94%           
=======================================
  Files         137      137           
  Lines       23475    23475           
=======================================
+ Hits        18295    18297    +2     
+ Misses       5180     5178    -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5157?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5157/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `75.00% <0.00%> (-0.41%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5157/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.96% <0.00%> (-0.15%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5157/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.90% <0.00%> (+1.38%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5157?src=pr&el=footer). Last update [68e19f1...49b1e42](https://codecov.io/gh/huggingface/transformers/pull/5157?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
