[TF generate] Cut encoder outptus to just last hidden states for now
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Fixes #8361

As shown in #8361, the current TF generate can throw errros if `config.output_attentions` is set to `True`. Because we cannot outptut `attentions` and `hidden_states` at the moment anyways in TF, this PR just cuts the encoder outputs to just the `last_hidden_states`

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