[s2s] add support for overriding config params
Author: stas00
Pull request: yes

add support for overriding model params:
```
python finetune.py --encoder_layerdrop 0.1 --decoder_layerdrop 0.1 --dropout 0.1 --attention_dropout 0.1
```
as requested at  https://github.com/huggingface/transformers/issues/6018

`README.md` seems to be mostly the editor removing superfluous whitespace - not sure why github shows it - normally it doesn't. The only added doc section is https://github.com/stas00/transformers/blob/seq2seq-train_params-1/examples/seq2seq/README.md#finetuning-training-params



Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6149?src=pr&el=h1) Report
> Merging [#6149](https://codecov.io/gh/huggingface/transformers/pull/6149?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/54f9fbeff822ec0547fd23d0338654456925f6b7&el=desc) will **increase** coverage by `1.32%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6149/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6149?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6149      +/-   ##
==========================================
+ Coverage   78.35%   79.68%   +1.32%     
==========================================
  Files         146      146              
  Lines       26403    26403              
==========================================
+ Hits        20689    21039     +350     
+ Misses       5714     5364     -350     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6149?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6149/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `24.22% <0.00%> (-63.98%)` | :arrow_down: |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6149/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `60.56% <0.00%> (-35.22%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/6149/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `65.42% <0.00%> (-29.91%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6149/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `85.71% <0.00%> (-0.51%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6149/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.20% <0.00%> (-0.29%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6149/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `98.79% <0.00%> (+34.61%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/6149/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `96.77% <0.00%> (+73.38%)` | :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/6149?src=pr&el=footer). Last update [54f9fbe...5476a9e](https://codecov.io/gh/huggingface/transformers/pull/6149?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


The code looks perfect.

> I'm surprised you didn't need to change the `CHEAP_ARGS` constant in the tests.

because the new args are optional? Unless you mean something else.

...Working on the tests. 

Added tests as suggested. 

good alias
```bash
sty () {
	make style
	flake8 examples templates tests src utils
}
```