[EncoderDecoderModel] add a add_cross_attention boolean to config (#6377)

* correct encoder decoder model

* Apply suggestions from code review

* apply sylvains suggestions

src/transformers/configuration_encoder_decoder.py

@@ -58,6 +58,7 @@ class EncoderDecoderConfig(PretrainedConfig):
             >>> config_decoder  = model.config.decoder
             >>> # set decoder config to causal lm
             >>> config_decoder.is_decoder = True
+            >>> config_decoder.add_cross_attention = True
 
             >>> # Saving the model, including its configuration
             >>> model.save_pretrained('my-model')
@@ -94,8 +95,9 @@ class EncoderDecoderConfig(PretrainedConfig):
         Returns:
             :class:`EncoderDecoderConfig`: An instance of a configuration object
         """
-        logger.info("Set `config.is_decoder=True` for decoder_config")
+        logger.info("Set `config.is_decoder=True` and `config.add_cross_attention=True` for decoder_config")
         decoder_config.is_decoder = True
+        decoder_config.add_cross_attention = True
 
         return cls(encoder=encoder_config.to_dict(), decoder=decoder_config.to_dict())