Delete Model2Model
- the quickstart code doesn't work
- the tests don't test a forward pass

if you need it `git checkout e8ce63ff`
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3019?src=pr&el=h1) Report
> Merging [#3019](https://codecov.io/gh/huggingface/transformers/pull/3019?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e8ce63ff2163259276fc0a4a2f35b836fe9f4aa0?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3019/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3019?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3019      +/-   ##
==========================================
+ Coverage   77.25%   77.27%   +0.01%     
==========================================
  Files          98       98              
  Lines       16040    16030      -10     
==========================================
- Hits        12392    12387       -5     
+ Misses       3648     3643       -5
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3019?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/3019/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbmNvZGVyX2RlY29kZXIucHk=) | `21.05% <ø> (-4.33%)` | :arrow_down: |
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/3019/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `98.91% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3019?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/3019?src=pr&el=footer). Last update [e8ce63f...835a807](https://codecov.io/gh/huggingface/transformers/pull/3019?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Hi, why is the Model2Model deleted? I am using it and it works great for me. <|||||>It was not super well documented or tested and we didn't want to maintain it. If you're interested in sending a PR with working quickstart code and tests (could start by reverting this) we would definitely be happy to add it back! Sorry!