Fix issue #7781
`decoder_config` may not be defined at this line, use `kwargs_decoder["config"]` instead. This is also consistent with the rest of the function.

Fixes # 7781
https://github.com/huggingface/transformers/issues/7781

## Who can review?

Anybody can review, though @patrickvonplaten gave an informal approval in the issue discussion.
Hey @jsilter - sorry this fix was already pushed yesterday in #7903. Thanks for spotting the bug!!!