GitLab

* Addressed review issues in PR #668

* Changed GTZAN so that it only traverses filenames belonging to the dataset

Now, instead of walking the whole directory and subdirectories of the dataset
GTZAN only looks for files under a `genre`/`genre`.`5 digit number`.wav format, where `genre` is an allowed GTZAN genre label.
This allows moving or removing files from the dataset (e.g. for fixing duplication or mislabeling issues).

* Added the popular GTZAN dataset:

* Added the GTZAN class in torchaudio.datasets using the same format as the rest of the datasets.
* Added the appropriate test function in test_datasets.py.
* Added the GTZAN class in the datasets.rst documentation file.

* Addressed review issues in PR #668

* Added dummy noise .wav in `test/assets/`
* Removed transforms of input and output from the dataset
  `__init__` function, as well as the corresponding methods.
* Replaced rendundant `filtered` and `subset` methods from
  class initialization and also changed the corresponding
  assertion message.

* Fixed E303: too many blank lines error

* Added GTZAN to __init__.__all__

* Fixed incorrectly not importing GTZAN

* removed duplicate warning

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