Add benchmark notebook
This PR adds a general notebook on how to use the Benchmark tools
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5312?src=pr&el=h1) Report
> Merging [#5312](https://codecov.io/gh/huggingface/transformers/pull/5312?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/135791e8ef12802ceb21a4abbb3a93f7da1bf390&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5312/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5312?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5312   +/-   ##
=======================================
  Coverage   79.30%   79.30%           
=======================================
  Files         138      138           
  Lines       24283    24283           
=======================================
  Hits        19258    19258           
  Misses       5025     5025           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5312?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5312/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.62% <0.00%> (-0.15%)` | :arrow_down: |
| [src/transformers/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/5312/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190cmFuc2ZvX3hsLnB5) | `78.93% <0.00%> (+0.19%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5312?src=pr&el=footer). Last update [135791e...487fa69](https://codecov.io/gh/huggingface/transformers/pull/5312?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
