Add NER TF2 example.
Author: jplu
Pull request: yes

Create a NER example similar to the Pytorch one. It takes the same options, and can be run the same way.

As you asked @julien-c I prefered I did a fresh new PR :)

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2046?src=pr&el=h1) Report
> Merging [#2046](https://codecov.io/gh/huggingface/transformers/pull/2046?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7edb51f3a516ca533797fb2bb2f2b7ce86e0df70?src=pr&el=desc) will **increase** coverage by `0.05%`.
> The diff coverage is `78.17%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2046/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2046?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2046      +/-   ##
==========================================
+ Coverage   83.45%   83.51%   +0.05%     
==========================================
  Files         105      107       +2     
  Lines       15568    15765     +197     
==========================================
+ Hits        12993    13166     +173     
- Misses       2575     2599      +24
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2046?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/2046/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX2Rpc3RpbGJlcnQucHk=) | `95.22% <26.66%> (-3.44%)` | :arrow_down: |
| [transformers/optimization\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/2046/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL29wdGltaXphdGlvbl90Zi5weQ==) | `79.82% <79.82%> (ø)` | |
| [transformers/tests/optimization\_tf\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2046/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL29wdGltaXphdGlvbl90Zl90ZXN0LnB5) | `86.76% <86.76%> (ø)` | |
| [transformers/tests/modeling\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2046/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX2NvbW1vbl90ZXN0LnB5) | `74.23% <0%> (+0.19%)` | :arrow_up: |
| [transformers/tests/modeling\_bert\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2046/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX2JlcnRfdGVzdC5weQ==) | `96.5% <0%> (+0.5%)` | :arrow_up: |
| [transformers/tests/modeling\_xlnet\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2046/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3hsbmV0X3Rlc3QucHk=) | `96.12% <0%> (+0.64%)` | :arrow_up: |
| [transformers/tests/modeling\_xlm\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2046/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3hsbV90ZXN0LnB5) | `96% <0%> (+0.66%)` | :arrow_up: |
| [transformers/tests/modeling\_roberta\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2046/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3JvYmVydGFfdGVzdC5weQ==) | `75.38% <0%> (+0.76%)` | :arrow_up: |
| ... and [11 more](https://codecov.io/gh/huggingface/transformers/pull/2046/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2046?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2046?src=pr&el=footer). Last update [7edb51f...9200a75](https://codecov.io/gh/huggingface/transformers/pull/2046?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


This looks great, amazing work @jplu!

Before merging we would need to add:
- a few tests on the optimizer (create a new file `./transformers/tests/optimization_tf_test.py` like in `./transformers/tests/optimization_test.py`)
- documentation for the optimizer (for instance in `./docs/source/main_classes/optimizer_schedules.rst.py`)
- an example of a command line to run the `run_tf_ner.py` script and the associated results you should obtain (in `./examples/README.md`)

Do you think you can do it?

Thanks a lots! :)

I can do these tasks, no problems!!

I have done what you asked @thomwolf, please let me know if I have to change something.

This is awesome, merging! 

Amazing!! Thanks a lot ;)