[GenerationOutputs] Fix GenerationOutputs Tests 
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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The `GenerationOutputs` PR: https://github.com/huggingface/transformers/pull/9150 was not rebased, so that the cicrle ci on master is red now. This PR fixes it.


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Comments:
This PR actually made me correct 2 bugs additionally:

1) past_key_values for BertForCausalLM
2) T5 should not return T5 cross attentions if just encoder model -> make sure encoder model has never `config.is_decoder=True`