[WIP][Examples] Adding more examples and more introductory tutorials
Author: thomwolf
Pull request: yes


Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5635?src=pr&el=h1) Report
> Merging [#5635](https://codecov.io/gh/huggingface/transformers/pull/5635?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/8edfaaa81b9995cedea2f8805e4c18c2b6cb5bfc&el=desc) will **decrease** coverage by `0.58%`.
> The diff coverage is `18.18%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5635/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5635?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5635      +/-   ##
==========================================
- Coverage   78.29%   77.71%   -0.59%     
==========================================
  Files         146      146              
  Lines       26607    26344     -263     
==========================================
- Hits        20832    20473     -359     
- Misses       5775     5871      +96     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5635?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/5635/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `83.85% <18.18%> (-9.99%)` | :arrow_down: |
| [...c/transformers/modeling\_tf\_transfo\_xl\_utilities.py](https://codecov.io/gh/huggingface/transformers/pull/5635/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90cmFuc2ZvX3hsX3V0aWxpdGllcy5weQ==) | `9.90% <0.00%> (-76.24%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/5635/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90cmFuc2ZvX3hsLnB5) | `17.22% <0.00%> (-72.24%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/5635/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `25.82% <0.00%> (-62.38%)` | :arrow_down: |
| [src/transformers/tokenization\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/5635/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxuZXQucHk=) | `66.66% <0.00%> (-21.63%)` | :arrow_down: |
| [src/transformers/tokenization\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5635/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcm9iZXJ0YS5weQ==) | `76.71% <0.00%> (-19.18%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/5635/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `80.98% <0.00%> (-11.99%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/5635/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `65.42% <0.00%> (-6.24%)` | :arrow_down: |
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/5635/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `38.73% <0.00%> (-3.76%)` | :arrow_down: |
| ... and [49 more](https://codecov.io/gh/huggingface/transformers/pull/5635/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5635?src=pr&el=footer). Last update [8edfaaa...67859e5](https://codecov.io/gh/huggingface/transformers/pull/5635?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Ooops, the rebase made the diff unreadable on this PR. Opening a new PR from this branch.