[Benchmark] Add benchmarks for TF Training
Author: patrickvonplaten
Pull request: yes

This PR adds train functions to TF benchmarks including tests.
The notebook is updated accordingly as well.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5594?src=pr&el=h1) Report
> Merging [#5594](https://codecov.io/gh/huggingface/transformers/pull/5594?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/cfbb98297449e09e5a2443b4ba76be52a71ec0f7&el=desc) will **increase** coverage by `1.13%`.
> The diff coverage is `15.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5594/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5594?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5594      +/-   ##
==========================================
+ Coverage   76.95%   78.08%   +1.13%     
==========================================
  Files         145      145              
  Lines       25317    25351      +34     
==========================================
+ Hits        19482    19795     +313     
+ Misses       5835     5556     -279     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5594?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/benchmark/benchmark\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/5594/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrX3RmLnB5) | `61.53% <10.81%> (-18.28%)` | :arrow_down: |
| [src/transformers/benchmark/benchmark.py](https://codecov.io/gh/huggingface/transformers/pull/5594/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrLnB5) | `74.01% <66.66%> (ø)` | |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5594/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `43.98% <0.00%> (-49.38%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5594/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `83.95% <0.00%> (-1.01%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5594/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `79.26% <0.00%> (-0.34%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5594/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.88% <0.00%> (+0.98%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5594/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `96.77% <0.00%> (+73.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5594?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/5594?src=pr&el=footer). Last update [cfbb982...2e17a73](https://codecov.io/gh/huggingface/transformers/pull/5594?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Merging. Pinging @LysandreJik for notification.