Fix ipynb generation (.md -> code block) (#862)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/862

Currently everything is a .md block, which looks fine in html, but doesn't translate properly to .ipynb

This diff fixes the rendering to work in both html and ipynb

- Pushed code into code cells
- Adding mocking https://github.com/sphinx-gallery/sphinx-gallery/pull/941 to get around runtime code not being run in Notebook
- Fixing inline code (.ipynb uses a different whitespace requirement than html sphinx)

Reviewed By: tarun292

Differential Revision: D50209589

fbshipit-source-id: deb1c2a8b52edce1783f042e10aa51bad7dd2516