Update: add using pcm bytes (#4323)
@lhoestq Maybe I'm missing something, but what's the reason to read and encode PCM files to WAV in `Audio.encode_example`. Isn't the whole purpose of the decodable types to operate on raw files whenever possible? IMO this PR should only modify `Audio.decode_example` to support PCM files/bytes decoding. Because the PCM file is not enough, we also need the `sampling_rate` associated to it. Therefore the two alternatives are either:
- convert to WAV
- add a `sampling_rate` field to the Audio arrow storage (not sure how it would behave for backward compatibility though) But [`scipy.io.wavfile.read`](https://docs.scipy.org/doc/scipy/reference/generated/scipy.io.wavfile.read.html), which is used for reading such files, returns a file's sampling rate. The only tricky part is [resampling](https://stackoverflow.com/questions/33682490/how-to-read-a-wav-file-using-scipy-at-a-different-sampling-rate) to a different sampling rate than the default one. How does it get the sampling rate of a PCM file then ? According to [SO](https://stackoverflow.com/a/57027667/17517845) it's not possible to infer it from the file alone > Awesome thanks ! Could you also add tests in `tests/features/test_audio.py` ?
> 
> Maybe add a small pcm file in `tests/features/data` and check that everything works as expected in tests cases like `test_audio_encode_example_pcm` and `test_audio_decode_example_pcm` for example.

@lhoestq  how can i test test_audio.py? where is "__main__" func?
do you have some example or guideline? > But [`scipy.io.wavfile.read`](https://docs.scipy.org/doc/scipy/reference/generated/scipy.io.wavfile.read.html), which is used for reading such files, returns a file's sampling rate. The only tricky part is [resampling](https://stackoverflow.com/questions/33682490/how-to-read-a-wav-file-using-scipy-at-a-different-sampling-rate) to a different sampling rate than the default one.

@mariosasko @lhoestq 
thanks for comment!

First of all, "PCM file" can not read alone to any audio library.
"PCM file" has not any audio META information header. (it just purely audio byte data. therefore, we don't have to encoding and decoding)
but, "PCM file" is audio extension, so we can use `datasets.Audio`

if you want to read "PCM file" to audio file likely, it have to needs additional parameter. (channel, sampling_rate, else....)
but, in many situation, we only know sampling_rate for PCM

and, if we want to use `datasets.Audio` for "PCM file", we must process encode_example.
therefore, i have to use sampling_rate for encoding for making wav-style byte. (we only know sampling_rate)

In my source code, I don't compare sampling rate(`datasets.Audio's self.sampling_rate` and `read pcm sampling_rate(value["sampling_rate"])`) and checking mono
@mariosasko ! do you want to process resampling and making mono? then i can modify my source
 There is no "main" function in test scripts :) To run a test script you must use the `pytest` command:
```
pytest tests/features/test_audio.py
```

to run only one function you can also do
```
pytest tests/features/test_audio.py::test_audio_feature_type_to_arrow
```
for example @lhoestq
maybe, if i write test code, i have to commit test_audio.py and send pr?
because, we need to keep `test_audio_encode_example_pcm` and `test_audio_decode_example_pcm` method after my pr merged? You can add your tests in this PR with the other changes you did @lhoestq 
test complete & commit my test_audio.py

AND, some change in my code.

audio.py
i think "sampling_rate" is already Audio object initial variable. so, we don`t have to use input parameter.

test_audio.py
we can check "PCM" file to path (exactly, extenstion)
so, test case has to know `path`. if only have `bytes`, we don`t know that is "PCM" or not @lhoestq
and, why circleci raised exception?
maybe, [repo](https://huggingface.co/api/datasets/lhoestq/_dummy?full=true) url is not found!
PLZ, CHK! @lhoestq
hello???? @lhoestq 
test_audio.py
if we don`t use path in pcm, test-case need to be changed
so, we check path just None i'm merge branch already and `multiprocess` in `setup.py` but circleci error only win version
![image](https://user-images.githubusercontent.com/34292279/175461714-c7d2e741-3b7b-40a3-bba9-13ce2af0055c.png)
how can i fixed it? @lhoestq thx for comment!
test_audio.py test complete. it runs sucessfully
and, self.get("sampling_rate") -> value.get("sampling_rate") changed

and, some comment is not agreed to me, plz check my sub comment! _The documentation is not available anymore as the PR was closed or merged._
first of all, please look #4323

why i can not use {"path","array","sampling_rate"}
because sf.write(format="wav") and sf.read(BytesIO) is changed my pcm data value
maybe, i think wav got header but, pcm is not.
and variable naming, pcm data is "byte" type. so, "array" name is not fair i think

so, i use scipy lib and numpy (that is huggingface dependency)
and refer to @lhoestq answered,
1. encode -> using sampling_rate and pcm byte -> wav style byte (scipy.wavfile.write to byte)
2. byte converting using fairseq style pcm audio read  [FileAudioDataset](https://github.com/facebookresearch/fairseq/blob/main/fairseq/data/audio/raw_audio_dataset.py)
4. decode -> read wavfile.read

that way is not screw up my pcm byte to float data, and another audio type(wav) safety

please check!