Use DataCollatorForSeq2Seq in run_summarization in all cases
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

Removes unnecessary `default_data_collator` import.


## 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: #10791
- [ ] Did you make sure to update the documentation with your changes? Here are the
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?
@sgugger Now ran make quality ;), thanks!

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Comments:
Thanks a lot!