GitLab

* Refactor tutorial organization

* Merge tutorial subdirectoris under to examples/gallery/tutorials
* Do not use index.rst generated by Sphinx-gallery
* Instead use flat structure so that all the tutorials are listed in left menu
* Use `_assets` dir for artifacts of tutorials

* Use cmake for third party

* Apply patch to libmad

* Update gitignore

* Update docker test image

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

.gitignore _ext/

vctk dataset

add label processing

added tests

chunk dataset

simple caching scheme

caching fixed

vctk downsample

yesno dataset