Google Assistant app has been uploaded to play store by Google and is now available to download. But it is just a shortcut which allows launching assistant app if it already available on your smartphone.
With this app, you can launch Google Assistant with a single tap on the assistant app icon installed from play store and no need to touch and hold the home button.
By having a separate app on play store, Google can able to provide frequent updates and other improvements to the assistant app. Hope for more improvements in future.
Download Google Assistant from Play store here.