Integration of TF 2.0 models with other Keras modules
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.
# [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).
