If you would like to re-install stock Android, you can follow the below steps.

Web Installer (Recommended, works for all Pixels)

Google provides a web installer for the Pixels, which means that you can directly install the stock OS from a web browser (even from another phone!) without having to install anything.

Going to https://flash.android.com/back-to-public will automatically select the latest version available from Google for your device and guide you through installation.

Factory images (Manual installation required)

These are the latest stock factory images for your device. You can download and install them manually.

Visit https://developers.google.com/android/images to get links to all versions. It also has installation instructions.