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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`.

* fix download

* fix reading tsv archive

* add new languages

* maintain same structure as other datasets

* update CommonVoice Tests

* fix

* change directory name

* remove extra line

* add speechcommand dataset and test

* prepend the full path to each result

* add missing param on docstring in walk_files

* add file to run tests on SpeechCommand Dataset

* reduce logic

* update test on SpeechCommands

* correct the indentation on docstring walk_files

* flake8 compliance

* change tuple type returned. move path split logic in load item.

* typo in name.

* redundant file path.

* filter background noise.
Co-authored-by: Vincent QB <vincentqb@users.noreply.github.com>

* LJ Speech dataset

* refactoring

as per @vincentqb's suggestions

* close file.

* staying with datapoints as tuples until further notice.

* loading tsv as dict.

* BackgroundGenerator
* renaming disk cache.

Fix test for datasets.

* new dataset format.
* add basic test.
* files for testing.
* serialization using torch.
* add diskcache.
* adding deprecation warnings.
* removing legacy.
* warning about transforms.
* detecting file format using reader.