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Resolves #1125 where dataset metadata does not contain an extension.

The use of `walk_files` made it ambiguous who is responsible to locate 
the correct set of files. (Dataset class? or utility?)
In fact, just glob-ing everything is not the right problem being solved in implementing
Dataset, because if you have a specific dataset you consider to access, then
the directory structure and file locations are determined. No need to do arbitral number of recursions.
Each Dataset implementation should be glob-ing the right set of files it requires.

* Removes code for download logics 
* [BC-breaking] Changes the meaning of `root` argument to the exact directory of the dataset
* Deprecates the constructor arguments for download and subdirectory construction

This reverts commit 09a6fca1.

no longer allow to download the dataset directly. deprecate: download and url. add language.

We have been running unit test with editable installation. (i.e. `python setup.py develop`), with which we missed issues like #842. 

This CC makes installation in CI non-editable, and change test directory structure so that the source code will not shadow the installed version of `torchaudio`. With simple `pytest test`, `pytest` modifies `sys.path` and prepend checked out repository, which shadows the installed version.

To remedy this, the whole test suites has been moved from `./test` to `./test/torchaudio_unittest`. This adds nice module structure to our test code and we can do absolute import in each test module, which makes it possible again to run test with `python -m unittest torchaudio_unittest/XXX.py`

This change does not affect the regular development process (`python setup.py develop` && `pytest test`)

* Add test for CommonVoice dataset

* Migrate the existing tests for `bg_iterator` and `diskcache_iterator` to `test/datasets/utils_test.py`
Co-authored-by: Leon Gao <legao@linkedin.com>