[Bart] Correct wrong order in shift token to right in Bart

[patrickvonplaten]

What does this PR do?

The previous PR #9131 implemented the replacement of -100 with pad_token after retrieving the eos_token_idx. However it should be done before to make sure the correct eos_token_id is found.

Thanks a lot @patil-suraj for spotting this.

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[patil-suraj]

LGTM!