[EncoderDecoderConfig] automatically set decoder config to decoder
When instantiating an encoder-decoder configuration from two pretrainied configs, the decoder config should automatically set to `config.is_decoder=True`. 

In general, whenever we instantiate an encoder decoder model, no matter how, the resulting decoder config should have the attribute `decoder.is_decoder=True`. 

This PR also adds a couple of tests to make sure that an encoder-decoder model can be instantiated from two configs over the encoder decoder config class.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4809?src=pr&el=h1) Report
> Merging [#4809](https://codecov.io/gh/huggingface/transformers/pull/4809?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/47a551d17b6ed2eaf03301f049006d559fca5cf3&el=desc) will **decrease** coverage by `1.41%`.
> The diff coverage is `0.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4809/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4809?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4809      +/-   ##
==========================================
- Coverage   77.14%   75.72%   -1.42%     
==========================================
  Files         128      128              
  Lines       21073    21075       +2     
==========================================
- Hits        16256    15959     -297     
- Misses       4817     5116     +299     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4809?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/4809/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2VuY29kZXJfZGVjb2Rlci5weQ==) | `35.71% <0.00%> (-2.75%)` | :arrow_down: |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4809/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `18.70% <0.00%> (-74.83%)` | :arrow_down: |
| [src/transformers/configuration\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4809/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2xvbmdmb3JtZXIucHk=) | `76.92% <0.00%> (-23.08%)` | :arrow_down: |
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/4809/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `77.18% <0.00%> (-6.35%)` | :arrow_down: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/4809/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `86.77% <0.00%> (-0.19%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4809/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.83% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4809/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.17% <0.00%> (+0.23%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/4809/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `81.78% <0.00%> (+1.37%)` | :arrow_up: |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/4809/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `86.21% <0.00%> (+14.10%)` | :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/4809?src=pr&el=footer). Last update [47a551d...6d8a589](https://codecov.io/gh/huggingface/transformers/pull/4809?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Merging for now since this is still unreleased code.<|||||>@LysandreJik - not sure what codecov complains about.