`decoder_config` used before intialisation (#7903)

Seeing error when sending `decoder_config` as a parameter while initializing a encoder-decoder model from pretrained. 
fixed "UnboundLocalError: local variable 'decoder_config' referenced before assignment"

src/transformers/modeling_encoder_decoder.py

@@ -327,7 +327,7 @@ class EncoderDecoderModel(PreTrainedModel):
 
                 kwargs_decoder["config"] = decoder_config
 
-            if kwargs_decoder["config"].is_decoder is False or decoder_config.add_cross_attention is False:
+            if kwargs_decoder["config"].is_decoder is False or kwargs_decoder["config"].add_cross_attention is False:
                 logger.warning(
                     f"Decoder model {decoder_pretrained_model_name_or_path} is not initialized as a decoder. In order to initialize {decoder_pretrained_model_name_or_path} as a decoder, make sure that the attributes `is_decoder` and `add_cross_attention` of `decoder_config` passed to `.from_encoder_decoder_pretrained(...)` are set to `True` or do not pass a `decoder_config` to `.from_encoder_decoder_pretrained(...)`"
                 )