Modelling Encoder-Decoder | Error :- `decoder_config` used before intialisation
Getting error when sending `decoder_config` as a parameter while initializing an encoder-decoder model from pretrained. 

# What does this PR do?
fixes "UnboundLocalError: local variable 'decoder_config' referenced before assignment"

## Who can review?
@patrickvonplaten @sgugger
@patrickvonplaten @sgugger, please review<|||||>Great catch @ayubSubhaniya !