[SMALL BREAKING CHANGE] Cleaning up configuration classes - Adding Model Cards
Clean up configuration.

Previously loading a JSON file in the configuration could be done either by `config = config_class(json_file)` or by `config = config_class.from_pretrained(json_file)`.

This was a historical artifact from the time configuration classes didn't use `from_pretrained()` method. This introduced complexity in logic to instantiate the classes which impacted PRs like #1548 and complexified the code to add new models. 

In this  PR we remove the first path to favor using the standardized `config = config_class.from_pretrained(json_file)`.

cc @LysandreJik @mfuntowicz @julien-c 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2164?src=pr&el=h1) Report
> Merging [#2164](https://codecov.io/gh/huggingface/transformers/pull/2164?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e92bcb7eb6c5b9b6ed313cc74abaab50b3dc674f?src=pr&el=desc) will **decrease** coverage by `0.99%`.
> The diff coverage is `90.53%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2164/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2164?src=pr&el=tree)

```diff
@@           Coverage Diff            @@
##           master    #2164    +/-   ##
========================================
- Coverage   81.35%   80.35%    -1%     
========================================
  Files         120      122     +2     
  Lines       18254    18335    +81     
========================================
- Hits        14851    14734   -117     
- Misses       3403     3601   +198
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2164?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/tests/modeling\_tf\_openai\_gpt\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2164/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX29wZW5haV9ncHRfdGVzdC5weQ==) | `94.73% <ø> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_bert\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2164/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2JlcnRfdGVzdC5weQ==) | `96.22% <ø> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_roberta\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2164/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX3JvYmVydGFfdGVzdC5weQ==) | `75.2% <ø> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_ctrl\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2164/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX2N0cmxfdGVzdC5weQ==) | `93.57% <ø> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2164/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX2NvbW1vbl90ZXN0LnB5) | `75.64% <ø> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_t5\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2164/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX3Q1X3Rlc3QucHk=) | `92.77% <ø> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_bert\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2164/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX2JlcnRfdGVzdC5weQ==) | `97.07% <ø> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_gpt2\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2164/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX2dwdDJfdGVzdC5weQ==) | `94.16% <ø> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_ctrl\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2164/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2N0cmxfdGVzdC5weQ==) | `94.05% <ø> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_distilbert\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2164/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2Rpc3RpbGJlcnRfdGVzdC5weQ==) | `99.08% <ø> (ø)` | :arrow_up: |
| ... and [50 more](https://codecov.io/gh/huggingface/transformers/pull/2164/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2164?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/2164?src=pr&el=footer). Last update [e92bcb7...1bbdbac](https://codecov.io/gh/huggingface/transformers/pull/2164?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>awesome<|||||>Ok merging