Shifting labels for causal LM when using label smoother
# What does this PR do?

Fixes #17960 

When training CausalLM such as GPT2, loss is computed within model's `foward()` function and `labels` are shifted internally. However, if label smoothing is applied, loss is computed in trainer's `compute_loss` function and `labels` are not shifted. This causes misalignment of `labels` and corresponding `input_ids`. This commit is for resolving this misalignment.




## Before submitting
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      Pull Request section?
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- [ ] Did you write any new necessary tests?


## Who can review?
@sgugger

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