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* Add util to generate whitenoise

* Use sinusoid for pitch and revert vad

* Introduce common utility for defining test matrix for device/dtype

* Make resample_waveform support float64

* Mark lfilter related test as xfail when float64

* fix

* Add safeguard to SoX initialization/shutdown

* Initialize SoX automatically when it's needed

This PR aims the following things;
1. Introduce and adopt helper function `get_asset_path` that abstract the logic to construct path to test assets.
2. Remove `create_temp_assets_dir` anywhere except `test_io`.

The benefits of doing so are,
a. the test code becomes simpler (no manual construction of asset path with `os.path.join`)
b. No unnecessary directory creation and file copies.

For 2. and b. tests in `test_io.py` (or tests that use `torchaudio.save`) are the only tests that need to write file to the disc, where the use of temporary directory makes it cleaner, therefore, `create_temp_assets_dir` is not necessary elsewhere. (still, `test_io` does not need to copy the entire asset directory, but that's not the point here.)

Also if any test is accidentally overwriting an asset data, not using a copy will make us aware of such behavior, so it is better to get rid of `create_temp_assets_dir`.

* testing with sox only when sox is available.

* use wav instead of mp3 for testing functions.

* typo.

* guard against not sox.

* backends definition.

* grouping backend functions into a separate file.

* remove duplicated code.

* requires sox.

* replace by wav, requires sox.

* require with scope.

* undo alignment.

* requires sox for these two, because of mp3.

* no longer need first mp3.

* cleaning.

* new wav version of file.

* flake8.

* remove unnecessary load.

* flake8.

* lint.

* lint.

* revert formatting of file.

* merging into common_utils.

* docstring.

* rename to common_utils.

- Apply __future__ imports uniformly (future division is the biggy, but absolute
  imports mattered too)
- Hotfix use of tempfile.TemporaryDirectory using a BC library (DO NOT
  add this library as a dependency to torchaudio; it's only for testing)
- Replace math.gcd with fractions.gcd
- Fix a weird pytest collection bug involving parametrized tests
- Turn on Python 2 and Python 3.7 in Travis.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>