Add task template for automatic speech recognition
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@SBrandeis @lhoestq i've integrated your suggestions, so this is ready for another review :) Merging if it's good for you @lewtun :)
This PR adds a task template for automatic speech recognition. In this task, the input is a path to an audio file which the model consumes to produce a transcription.

Usage:

```python
from datasets import load_dataset
from datasets.tasks import AutomaticSpeechRecognition

ds = load_dataset("timit_asr", split="train[:10]")
# Dataset({
#     features: ['file', 'text', 'phonetic_detail', 'word_detail', 'dialect_region', 'sentence_type', 'speaker_id', 'id'],
#     num_rows: 10
# })

task = AutomaticSpeechRecognition(audio_file_column="file", transcription_column="text")
ds.prepare_for_task(task)
# Dataset({
#     features: ['audio_file', 'transcription'],
#     num_rows: 10
# })
```
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