Align logits and labels in OPT
Author: MichelBartels
Pull request: yes

# What does this PR do?
For other decoder models, the labels are shifted and the last logit of each sequence is removed so they align when computing the loss. This isn't done for OPT. This PR adds this feature.
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Comments:
_The documentation is not available anymore as the PR was closed or merged._

LGTM - this is because we took the model from BART which is an encoder decoder
wdyt @ArthurZucker @patrickvonplaten ?

Oh, good find @MichelBartels! We weren't careful enough when reviewing the PR here - we should have aligned this with GPT2 right away.

The problem is that, it's not really wrong to **not** shift the labels and people could have already written their training pipelines with OPT where the labels are shifted before being passed to the model. So this could be backwards breaking here. I however do think it's important to align OPT as much as possible with GPT2. 

@LysandreJik @sgugger - do you think we could fix this in a patch release? 

Yes it needs to be addressed ASAP to avoid breaking changes, a patch release today is fine by me.

Yes, agreed! Let's merge this PR as-is, check if there are any other issues and do a patch release in a couple of hours.