Integration of TF 2.0 models with other Keras modules
Author: thomwolf
Pull request: yes

Add tests that TF 2.0 models can be integrated with other Keras modules.

Add more serialization tests for TF 2.0 and PyTorch models.

Fix TFSequenceSummary head and RoBERTa.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1482?src=pr&el=h1) Report
> Merging [#1482](https://codecov.io/gh/huggingface/transformers/pull/1482?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/6596e3d56626c921b3920e313866b7412633b91a?src=pr&el=desc) will **increase** coverage by `0.44%`.
> The diff coverage is `97.67%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1482/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1482?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1482      +/-   ##
==========================================
+ Coverage   85.59%   86.04%   +0.44%     
==========================================
  Files          91       91              
  Lines       13526    13566      +40     
==========================================
+ Hits        11578    11673      +95     
+ Misses       1948     1893      -55
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1482?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/1482/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX2Rpc3RpbGJlcnQucHk=) | `98.59% <ø> (+2%)` | :arrow_up: |
| [transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/1482/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX2dwdDIucHk=) | `94.79% <ø> (+1.31%)` | :arrow_up: |
| [transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/1482/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX29wZW5haS5weQ==) | `96.04% <ø> (+1.43%)` | :arrow_up: |
| [transformers/modeling\_tf\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/1482/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3RyYW5zZm9feGwucHk=) | `92.21% <ø> (+0.97%)` | :arrow_up: |
| [transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/1482/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX2JlcnQucHk=) | `96.6% <ø> (+0.89%)` | :arrow_up: |
| [transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1482/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3B5dG9yY2hfdXRpbHMucHk=) | `76.76% <0%> (-0.17%)` | :arrow_down: |
| [transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/1482/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2dwdDIucHk=) | `84.19% <100%> (+0.2%)` | :arrow_up: |
| [transformers/modeling\_tf\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/1482/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX2N0cmwucHk=) | `97.75% <100%> (+1.74%)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_xlnet\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1482/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX3hsbmV0X3Rlc3QucHk=) | `95.74% <100%> (+0.12%)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1482/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2NvbW1vbl90ZXN0LnB5) | `96.53% <100%> (+1.14%)` | :arrow_up: |
| ... and [12 more](https://codecov.io/gh/huggingface/transformers/pull/1482/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1482?src=pr&el=footer). Last update [6596e3d...4b8f3e8](https://codecov.io/gh/huggingface/transformers/pull/1482?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).