[Tests] Fix SpeechEncoderDecoder tests
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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This PR fixes the flaky tests:
- tests/test_modeling_speech_encoder_decoder.py::Wav2Vec2Speech2Text2::test_encoder_decoder_model_output_attentions
- tests/test_modeling_speech_encoder_decoder.py::Wav2Vec2BertModelTest::test_encoder_decoder_model_output_attentions
the problem was that for tests in 10% of the times layerdrop was used meaning that the attention_output was set to None. This PR corrects this by putting the model in `.eval()` mode.


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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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- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

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