Changes to I-BNN tutorial (#2889)

Summary:
## Motivation

* Take out comparison between methods. This makes the tutorial too long, and text descriptions of results can become inaccurate over time.
* Simplify code
* Put the MLP model that generates data into `eval` model (this changes results substantially)

Pull Request resolved: #2889

Test Plan: Looked at the notebook to make sure it rendered correctly

Reviewed By: saitcakmak

Differential Revision: D76934541

Pulled By: esantorella

fbshipit-source-id: 4a96a5d0dd228346e5aab55d33f1479894680a34

Author: esantorella