Fix GPT2 config set to trainable
There's currently a bug in the GPT2 model which prevents it from being saved. This is caused by setting the trainable parameter to the GPT2 config, which cannot be packaged later in the save pipeline. Gotta love python...

Here is a simple script which you can use to reproduce this bug (and check the fix):
```
from transformers import (TFGPT2Model)

if __name__ == '__main__':
    _base_model = TFGPT2Model.from_pretrained("gpt2")
    print(base_model._layers[0].trainable)
```
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2738?src=pr&el=h1) Report
> Merging [#2738](https://codecov.io/gh/huggingface/transformers/pull/2738?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/9e5b549b4d47678bdc74bc8f650e82cf25bfc245?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2738/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2738?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2738   +/-   ##
=======================================
  Coverage   74.09%   74.09%           
=======================================
  Files          93       93           
  Lines       15249    15249           
=======================================
  Hits        11298    11298           
  Misses       3951     3951
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2738?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/2738/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `94.66% <100%> (ø)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2738?src=pr&el=footer). Last update [9e5b549...5346295](https://codecov.io/gh/huggingface/transformers/pull/2738?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
