[Audio datasets] Adapting all audio datasets
@lhoestq - are there other important speech datasets that I'm forgetting here? 

Think PR is good to go otherwise @lhoestq @albertvillanova - how can we make an exception for the AMI README so that the test doesn't fail? The dataset card definitely should have a data preprocessing section Hi @patrickvonplaten ,

the data preprocessing section is not defined as a valid section in the readme validation file. After this line:
https://github.com/huggingface/datasets/blob/568db594d51110da9e23d224abded2a976b3c8c7/src/datasets/utils/resources/readme_structure.yaml#L20
feel free to insert (correctly indented of course):
```python
- name: "Dataset Preprocessing"
  allow_empty: true
  allow_empty_text: true
  subsections: null
```
and then the tests should pass. Thanks a lot @albertvillanova - I've added the feature to all audio datasets and corrected the task of `covost2`
This PR adds the new `Audio(...)` features - see: https://github.com/huggingface/datasets/pull/2324 to the most important audio datasets:

- Librispeech
- Timit
- Common Voice
- AMI
- ... (others I'm forgetting now)

The PR is curently blocked because the following leads to a problem:

```python
from datasets import load_dataset

# load first time works
ds = load_dataset("patrickvonplaten/librispeech_asr_dummy", "clean") 

# load from cache breaks
ds = load_dataset("patrickvonplaten/librispeech_asr_dummy", "clean") 
```

As soon as it's unblocked, I'll adapt the other audio datasets as well. 