[Wav2Vec2] Fix torch srcipt
Author: patrickvonplaten
Pull request: yes

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Fixes Wav2Vec2 torch script slow tests


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Comments:
It seems like `torch.trace(...)` doesn't like property function as it'll always call them. Since we've added the property as a private property in https://github.com/huggingface/transformers/pull/23813, let's just go the simplest way and change it to a function. 

This PR should fix: 
```
tests/models/wav2vec2/test_modeling_wav2vec2.py::Wav2Vec2ModelTest::test_torchscript_output_attentions
(line 1180)  ValueError: <class 'transformers.models.wav2vec2.modeling_wav2vec2.Wav2Vec2ForCTC'> has no adapter layers. Make sure to define config.adapter_attn_dim.
tests/models/wav2vec2/test_modeling_wav2vec2.py::Wav2Vec2ModelTest::test_torchscript_output_hidden_state
(line 1180)  ValueError: <class 'transformers.models.wav2vec2.modeling_wav2vec2.Wav2Vec2ForCTC'> has no adapter layers. Make sure to define config.adapter_attn_dim.
tests/models/wav2vec2/test_modeling_wav2vec2.py::Wav2Vec2ModelTest::test_torchscript_simple
(line 1180)  ValueError: <class 'transformers.models.wav2vec2.modeling_wav2vec2.Wav2Vec2ForCTC'> has no adapter layers. Make sure to define config.adapter_attn_dim.
tests/models/wav2vec2/test_modeling_wav2vec2.py::Wav2Vec2RobustModelTest::test_torchscript_output_attentions
(line 1180)  ValueError: <class 'transformers.models.wav2vec2.modeling_wav2vec2.Wav2Vec2ForCTC'> has no adapter layers. Make sure to define config.adapter_attn_dim.
tests/models/wav2vec2/test_modeling_wav2vec2.py::Wav2Vec2RobustModelTest::test_torchscript_output_hidden_state
(line 1180)  ValueError: <class 'transformers.models.wav2vec2.modeling_wav2vec2.Wav2Vec2ForCTC'> has no adapter layers. Make sure to define config.adapter_attn_dim.
tests/models/wav2vec2/test_modeling_wav2vec2.py::Wav2Vec2RobustModelTest::test_torchscript_simple
(line 1180)  ValueError: <class 'transformers.models.wav2vec2.modeling_wav2vec2.Wav2Vec2ForCTC'> has no adapter layers. Make sure to define config.adapter_attn_dim.
```

of the slow tests, such as
```
RUN_SLOW=1 pytest tests/models/wav2vec2/test_modeling_wav2vec2.py::Wav2Vec2ModelTest::test_torchscript_simple
```

@sgugger @ydshieh 

_The documentation is not available anymore as the PR was closed or merged._

One possible different way is to raise `AttributeError` instead of `ValueError` if we want to keep the propery.