Adding a Colab Notebook for ease of use

[sayakpaul]

This PR adds a Colab Notebook in order to make it easier for people to try out your awesome work easily.

Note that I have just added an explicit device placement in the Grad Rollout code. That's the only change in terms of code. The Colab Notebook mostly follows code from your vit_explain.py script.

If the PR gets merged the Colab Notebook needs to be updated to accommodate for the following:

• I am currently cloning the forked repository to reflect the device placement code update.
• Colab Notebook link.

[sayakpaul]

@jacobgil just checking to see if you had the time to consider this.