add example for Model2Model in quickstart
Author: rlouf
Pull request: yes

As per discussed @LysandreJik 

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2235?src=pr&el=h1) Report
> Merging [#2235](https://codecov.io/gh/huggingface/transformers/pull/2235?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ff36e6d8d713901af807719fa604518c451ff2e5?src=pr&el=desc) will **decrease** coverage by `1.09%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2235/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2235?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master    #2235     +/-   ##
=========================================
- Coverage   81.42%   80.32%   -1.1%     
=========================================
  Files         122      122             
  Lines       18348    18344      -4     
=========================================
- Hits        14940    14735    -205     
- Misses       3408     3609    +201
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2235?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2235/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3B5dG9yY2hfdXRpbHMucHk=) | `9.15% <0%> (-80.92%)` | :arrow_down: |
| [transformers/tests/modeling\_tf\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2235/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2NvbW1vbl90ZXN0LnB5) | `80.17% <0%> (-17.25%)` | :arrow_down: |
| [transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2235/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3JvYmVydGEucHk=) | `59.41% <0%> (-12.36%)` | :arrow_down: |
| [transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2235/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3hsbmV0LnB5) | `72.21% <0%> (-2.33%)` | :arrow_down: |
| [transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2235/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2N0cmwucHk=) | `94.27% <0%> (-2.21%)` | :arrow_down: |
| [transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2235/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX29wZW5haS5weQ==) | `80.19% <0%> (-1.33%)` | :arrow_down: |
| [transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/2235/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2RhdGEvcHJvY2Vzc29ycy9zcXVhZC5weQ==) | `14.18% <0%> (ø)` | :arrow_up: |
| [transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2235/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2ZpbGVfdXRpbHMucHk=) | `71.42% <0%> (+0.06%)` | :arrow_up: |
| [transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2235/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3V0aWxzLnB5) | `91.46% <0%> (+1.17%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2235?src=pr&el=footer). Last update [ff36e6d...a3245dd](https://codecov.io/gh/huggingface/transformers/pull/2235?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).