[Generation] Fix bug for manual decoder_input_ids + warning message
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Some improvements on the design of how `decoder_input_ids` are extracted that solve: https://github.com/huggingface/transformers/issues/9400
Also adds a nicer warning to prevent non-understandable errors as shown in: https://github.com/huggingface/transformers/issues/9464


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Comments:
Also checked that slow tests are passing