Encoder decoder config docs
As discussed on #5826, this PR adds more details on how to load encoder/decoder config objects from pretrained folders and how to instantiate encoder_decoder pretrained models given their corresponding configuration objects (useful for loading pre-trained models and modifying some config members for fine-tuning).


# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6195?src=pr&el=h1) Report
> Merging [#6195](https://codecov.io/gh/huggingface/transformers/pull/6195?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d8dbf3b75d58667e2ecaf42b4aa076e83d034d26&el=desc) will **increase** coverage by `0.32%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6195/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6195?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6195      +/-   ##
==========================================
+ Coverage   79.47%   79.80%   +0.32%     
==========================================
  Files         146      146              
  Lines       26607    26607              
==========================================
+ Hits        21146    21233      +87     
+ Misses       5461     5374      -87     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6195?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/6195/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2VuY29kZXJfZGVjb2Rlci5weQ==) | `100.00% <ø> (ø)` | |
| [src/transformers/modeling\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/6195/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbmNvZGVyX2RlY29kZXIucHk=) | `92.20% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6195/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `24.22% <0.00%> (-63.98%)` | :arrow_down: |
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6195/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `84.52% <0.00%> (-10.72%)` | :arrow_down: |
| [src/transformers/tokenization\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/6195/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxuZXQucHk=) | `90.09% <0.00%> (+23.42%)` | :arrow_up: |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6195/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `93.80% <0.00%> (+25.66%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6195/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `98.79% <0.00%> (+34.61%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6195?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/6195?src=pr&el=footer). Last update [d8dbf3b...4a03156](https://codecov.io/gh/huggingface/transformers/pull/6195?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>@afcruzs - thanks a lot for your PR. I changed the examples a bit trying to make sure:
1) Examples in `config` only concern the EncoderDecoderConfig
2) Examples in `model` only concern the EncoderDecoderModel

sorry for meddling in your PR.<|||||>Thanks for improving the examples! I've fixed the whitespace issue