Encoder decoder config docs (#6195)

* Adding docs for how to load encoder_decoder pretrained model with individual config objects

* Adding docs for loading encoder_decoder config from pretrained folder

* Fixing  W293 blank line contains whitespace

* Update src/transformers/modeling_encoder_decoder.py

* Update src/transformers/modeling_encoder_decoder.py

* Update src/transformers/modeling_encoder_decoder.py

* Apply suggestions from code review

model file should only show examples for how to load save model

* Update src/transformers/configuration_encoder_decoder.py

* Update src/transformers/configuration_encoder_decoder.py

* fix space

Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

src/transformers/configuration_encoder_decoder.py

@@ -56,6 +56,15 @@ class EncoderDecoderConfig(PretrainedConfig):
             >>> # Accessing the model configuration
             >>> config_encoder = model.config.encoder
             >>> config_decoder  = model.config.decoder
+            >>> # set decoder config to causal lm
+            >>> config_decoder.is_decoder = True
+
+            >>> # Saving the model, including its configuration
+            >>> model.save_pretrained('my-model')
+
+            >>> # loading model and config from pretrained folder
+            >>> encoder_decoder_config = EncoderDecoderConfig.from_pretrained('my-model')
+            >>> model = EncoderDecoderModel.from_pretrained('my-model', config=encoder_decoder_config)
     """
     model_type = "encoder_decoder"