[Finetune Seq2Seq Trainer] fix bert2bert test
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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This PR fixes the slow test: `tests/test_trainer_seq2seq.py::Seq2seqTrainerTester::test_finetune_bert2bert` which was failing because `rouge_score` was not added to the dependencies. In this PR I remove the usage of `datasets.load("rouge")` since it is very much unnecessary here => we are only testing whether training does not throw an error and for this it doesn't matter whether we use the rouge metric or accuracy. Removing `rouge` removes a dependency, which is the better way here IMO.


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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
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- tensorflow: @jplu

Library:

- benchmarks: @patrickvonplaten
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- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

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Comments:
failing test is unrelated -> waiting for @sgugger's approval before merging though.