Fix some doc examples in task summary
# What does this PR do?

Fix some doctest CI failure found [here](https://github.com/huggingface/transformers/runs/5877708951?check_suite_focus=true)

A few failures:

- 
    ```
    Expected nothing
    Got:
        '_unknown_'
    ```
- 
    ```
    Expected nothing
    Got:
        'MISTER QUILTER IS THE APOSTLE OF THE MIDDLE CLASSES AND WE ARE GLAD TO WELCOME HIS GOSPEL'
    ```
- 
    ```
    Expected:
        <pad> prosecutors say the marriages were part of an immigration scam. if convicted, barrientos faces two criminal
        counts of "offering a false instrument for filing in the first degree" she has been married 10 times, nine of them
        between 1999 and 2002.
    Got:
        <pad> prosecutors say the marriages were part of an immigration scam. if convicted, barrientos faces two criminal counts of "offering a false instrument for filing in the first degree" she has been married 10 times, nine of them between 1999 and 2002.</s><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad><pad>
    ```

There is one error I haven't fixed yet
```
No such file or directory: 'jfk_moon_speech.wav'
```

Do you know where to get this file for testing purpose ..?
_The documentation is not available anymore as the PR was closed or merged._<|||||>> Thanks for fixing these!
> 
> The `jfk_moon_speech.wav` is a local file I used to test some code examples, so maybe we can replace it with an audio example from the Hub?

Sure! Let me try. Haven't tried audio dataset from the Hub.<|||||>@sgugger @stevhliu I was able to use a file from dataset.

One example of change is

```py
>>> from transformers import pipeline
>>> from datasets import load_dataset
>>> import torch

>>> torch.manual_seed(42)  # doctest: +IGNORE_RESULT

>>> dataset = load_dataset("hf-internal-testing/librispeech_asr_demo", "clean", split="validation")
>>> dataset = dataset.sort("id")
>>> audio_file = dataset[0]["audio"]["path"]

>>> audio_classifier = pipeline(
...     task="audio-classification", model="ehcalabres/wav2vec2-lg-xlsr-en-speech-emotion-recognition"
... )
>>> predictions = audio_classifier(audio_file)
>>> predictions = [{"score": round(pred["score"], 4), "label": pred["label"]} for pred in predictions]
>>> predictions
[{'score': 0.1315, 'label': 'calm'}, {'score': 0.1307, 'label': 'neutral'}, {'score': 0.1274, 'label': 'sad'}, {'score': 0.1261, 'label': 'fearful'}, {'score': 0.1242, 'label': 'happy'}]
```
I ran the tests, and `task_summary.mdx` is fine, except the one in #16644 (it is not merged yet).

Let me know if you have further comments :-)