[TF Led] Fix wrong decoder attention mask behavior
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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This PR fixes TF LED. I wrongly added some lines to TFLed that automatically change the attention mask. However, this is incorrect behavior and not present in the PT version of the model. Sadly, I discovered this now after the release yesterday. @LysandreJik do you think we can patch this fix to circumvent breaking backward compatibility (but it's a bug IMO anyway).

This also fixes consequencetly the flaky `let_pt_tf_equivalence` test. I ran the test 40 times and it does not fail anymore.
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