Run summarization always use data collator for seq2 seq
Author: elsanns
Pull request: yes

# What does this PR do?

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Fixes #10791
This PR uses an instance of DataCollatorForSeq2Seq as a data collator regardless of the value of `pad_to_max_length`.
It fixes the problem of the script breaking with the two parameters set:
- label_smoothing_factor
- pad_to_max_length


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Discussion: https://github.com/huggingface/transformers/issues/10791
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
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## Who can review?
@sgugger


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