Support m4a format in torchaudio

[vincentqb]

We would like to support m4a format directly in torchaudio.

• This is the format used in the VoxCeleb2 dataset.
• It is supported by ffmpeg.

cc #104, @mravanelli

[faroit]

@vincentqb Without having looked at the ffmpeg io module... does it support multiple streams? If no, can this be supported?

See https://github.com/faroit/stempeg

[mthrok]

It does. You can map pick/duplicates source streams into arbitral combinations of output streams.

https://pytorch.org/audio/stable/tutorials/streaming_api_tutorial.html#example

[faroit]

It does. You can map pick/duplicates source streams into arbitral combinations of output streams.

https://pytorch.org/audio/stable/tutorials/streaming_api_tutorial.html#example

Oh that's cool. I can finally retire stempeg