Process .opus files with torchaudio
@anton-l suggested to proccess .opus files with `torchaudio` instead of `soundfile` as it's faster: 
![opus](https://user-images.githubusercontent.com/16348744/152177816-2df6076c-f28b-4aef-a08d-b499b921414d.png)

(moreover, I didn't manage to load .opus files with `soundfile` / `librosa` locally on any my machine anyway for some reason, even with `ffmpeg` installed).

For now my current changes work with locally stored file:
```python
# download sample opus file (from MultilingualSpokenWords dataset)
!wget https://huggingface.co/datasets/polinaeterna/test_opus/resolve/main/common_voice_tt_17737010.opus 

from datasets import Dataset, Audio

audio_path = "common_voice_tt_17737010.opus"
dataset = Dataset.from_dict({"audio": [audio_path]}).cast_column("audio", Audio(48000))
dataset[0]
# {'audio': {'path': 'common_voice_tt_17737010.opus',
#   'array': array([ 0.0000000e+00,  0.0000000e+00,  3.0517578e-05, ...,
#          -6.1035156e-05,  6.1035156e-05,  0.0000000e+00], dtype=float32),
#   'sampling_rate': 48000}}
```
But it doesn't work when loading inside s dataset from bytes (I checked on [MultilingualSpokenWords](https://github.com/huggingface/datasets/pull/3666), the PR is a draft now, maybe the bug is somewhere there  )

```python
import torchaudio
with open(audio_path, "rb") as b:
    print(torchaudio.load(b))
# RuntimeError: Error loading audio file: failed to open file <in memory buffer>
```