DOC: remove pytorch theme (because it is broken and unmaintained)

Author: mgeier

doc/usage.ipynb

@@ -416,10 +416,6 @@
     "  [example](https://nbsphinx.readthedocs.io/en/python-docs-theme/),\n",
     "  [usage](https://github.com/spatialaudio/nbsphinx/compare/python-docs-theme^...python-docs-theme)\n",
     "\n",
-    "* [pytorch_sphinx_theme](https://github.com/shiftlab/pytorch_sphinx_theme):\n",
-    "  [example](https://nbsphinx.readthedocs.io/en/pytorch-theme/),\n",
-    "  [usage](https://github.com/spatialaudio/nbsphinx/compare/pytorch-theme^...pytorch-theme)\n",
-    "\n",
     "* [sizzle](https://docs.red-dove.com/sphinx_sizzle_theme/):\n",
     "  [example](https://nbsphinx.readthedocs.io/en/sizzle-theme/),\n",
     "  [usage](https://github.com/spatialaudio/nbsphinx/compare/sizzle-theme^...sizzle-theme)\n",