[Flaky Generation Tests] Make sure that no early stopping is happening for beam search
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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The PR fixes the flaky ci which is probably due to early stopping in beam search with the top `num_return_sequences` beams being smaller than the 
longest `num_beams` beam.

This PR should (hopefully) fix flaky CI failures, such as:
- https://app.circleci.com/pipelines/github/huggingface/transformers/18912/workflows/70862bd9-bc94-4f2d-9b07-b85be146c867/jobs/155831
- https://app.circleci.com/pipelines/github/huggingface/transformers/18910/workflows/284df8e3-373b-4168-bc3b-f7079c5aa17d/jobs/155797


## 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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