Encoder decoder config docs

[afcruzs]

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]

Codecov Report

Merging #6195 into master will increase coverage by 0.32%.
The diff coverage is n/a.

@@            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	Coverage Δ
src/transformers/configuration_encoder_decoder.py	100.00% <ø> (ø)
src/transformers/modeling_encoder_decoder.py	92.20% <ø> (ø)
src/transformers/modeling_tf_flaubert.py	24.22% <0.00%> (-63.98%)	⬇️
src/transformers/tokenization_xlm_roberta.py	84.52% <0.00%> (-10.72%)	⬇️
src/transformers/tokenization_xlnet.py	90.09% <0.00%> (+23.42%)	⬆️
src/transformers/tokenization_marian.py	93.80% <0.00%> (+25.66%)	⬆️
src/transformers/modeling_tf_distilbert.py	98.79% <0.00%> (+34.61%)	⬆️

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[patrickvonplaten]

Example of the model can be simplified => no need to set decoder = True, since this is done automatically.

[patrickvonplaten]

@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.

[afcruzs]

Thanks for improving the examples! I've fixed the whitespace issue