Fix scheduled tests for `SpeechEncoderDecoderModel`
```
FAILED tests/test_modeling_speech_encoder_decoder.py::Wav2Vec2BertModelTest::test_real_model_save_load_from_pretrained
FAILED tests/test_modeling_speech_encoder_decoder.py::Speech2TextBertModelTest::test_real_model_save_load_from_pretrained
FAILED tests/test_modeling_speech_encoder_decoder.py::Wav2Vec2Speech2Text2::test_real_model_save_load_from_pretrained
```

**Problem**
`test_real_model_save_load_from_pretrained` was using a missing `self.get_inputs()` and failing for all three flavors of encoder-decoder models. CI logs: https://github.com/huggingface/transformers/actions/runs/1199765292

**Solution**
Pass pretrained models together with suitably-shaped inputs.