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This commit changes all the `torch.hub` network utility functions to
be imported from `torchaudio._internal`, so that later we can replace
the function within fbcode.

After #1966, `git submodule` is performed automatically.

Following the plan #1337, this commit drops the support for pseudo complex type from `F.phase_vocoder` and `T.TimeStretch`.

Following the plan #1337, this commit drops the support for pseudo complex type from 
`F.spectrogram` and `T.Spectrogram`.

It also deprecates the use of `return_complex` argument.

This commit changes the build process so that the third party code are fetched automatically when `setup.py` is invoked.

This is to allow
1. `build_sdist` to contain the third party library codes, so that source code distribution created at the time release tag is created is complete.
2. It makes it possible to do `pip install https://github.com/pytorch/audio.git`.
    Example:
    ```
    !pip install 'cmake>=3.18' ninja
    !pip install --verbose git+https://github.com/pytorch/audio.git@auto-source
    ```

This reverts commit 0822fc05.

Include the list of labels.

* Add wav2vec2 ASR German pretrained model from voxpopuli

In Wav2Vec2 ASR pipelines, the `get_model` method performs on-the-fly model
surgery to remove unused dimensions common to all the Wav2Vec2 model trained
with fairseq.

In VoxPopuli, there seems to be an extra dimensions introduced due to
some issue in the preprocessing stage.

For example, the label `1` shows up in the training dataset of German (1 out of 16M),
English (1 / 28M), Spanish (1 / 9.4M), Romanian (1 / 4.7M) and Polish (6 / 5.8M).

This code changes will allow the customization of excluded dimensions for such cases.

- Make the test support other languages
- Fetch tetst asset on-the-fly

It seems that this is no longer necessary for recent macOS .