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* 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

This commit adds colab/download/source link to tutorials, like in `pytorch/tutorials` repo.

Since the upstream `pytorch-sphinx-theme` does not provide the interface for this,
a hack to overwrite the URL is added.

This hack might stop working if there is some update in `pytorch-sphinx-theme`.