[SpeechEncoderDecoder] Fix from pretrained

[patrickvonplaten]

What does this PR do?

Fixes an edge case that leads to failing CI for: tests/test_modeling_speech_encoder_decoder.py::Wav2Vec2BertModelTest::test_real_model_save_load_from_pretrained

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[patrickvonplaten]

The other two VisionEncoderDecoderModel and EncoderDecoderModel were correct