[Seq2Seq] Correct import in Seq2Seq Trainer
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

Correct import as mentioned by @stas00 here: https://github.com/huggingface/transformers/pull/8194#discussion_r515690821

Pinging @stas00 for review as well here.
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## 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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 albert, bert, XLM: @LysandreJik
 GPT2: @LysandreJik, @patrickvonplaten
 tokenizers: @mfuntowicz
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 Benchmarks: @patrickvonplaten
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 nlp datasets: [different repo](https://github.com/huggingface/nlp)
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 Text Generation: @patrickvonplaten, @TevenLeScao
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 T5: @patrickvonplaten
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