[SpeechEncoderDecoder] Make sure no EOS is generated in test
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Fixes the flaky test: 

```tests/test_modeling_speech_encoder_decoder.py::Wav2Vec2Speech2Text2::test_encoder_decoder_model_generate```


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Models:

- albert, bert, xlm: @LysandreJik
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Comments:
_The documentation is not available anymore as the PR was closed or merged._

The reason for the flakiness comes from this recent change in `generate()`. 

Thanks for fixing!