Add frontmatter in notebooks

[ydcjeff]

To make ease of converting notebook to markdown files, add a few frontmatter in the notebook as html comments.

Current frontmatter to add in a cell (this should be at the most top of the notebook)

<!-- ---
title: Example title
description: Example description
date: 2021-07-27
downloads: true
include_footer: true
sidebar: true
tags:
  - deep learning
  - machine learning
  - pytorch
  - python
--- -->

cc @trsvchn @Priyansi

[vfdev-5]

@ydcjeff are we done with this issue and close it ?

[ydcjeff]

Yes, we now have frontmatters in every notebook.