Better TensorFlow 2 examples
This PR aims to improve the TensorFlow examples of the library. It aims to use many tf-specific paradigms, including but not limited to Keras API, AMP, XLA, `tensorflow_datasets`, `tf.train.Features` and `tf.train.Examples`, model saving using Keras callbacks.

Two examples are specifically targeted:

- [x]  `run_tf_glue.py`
- [ ]  `run_tf_squad.py`

Aims to have a similar architecture to the PyTorch examples: using a parser with similar arguments so as to be used from a terminal.

Feedback is most welcome.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1861?src=pr&el=h1) Report
> Merging [#1861](https://codecov.io/gh/huggingface/transformers/pull/1861?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a5a06a851e1da79138e53978aa079a093f243dde?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1861/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1861?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1861   +/-   ##
=======================================
  Coverage   81.43%   81.43%           
=======================================
  Files         122      122           
  Lines       18338    18338           
=======================================
  Hits        14933    14933           
  Misses       3405     3405
```



------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1861?src=pr&el=footer). Last update [a5a06a8...0f730de](https://codecov.io/gh/huggingface/transformers/pull/1861?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
