Setup and Usage of BoTorchModel in Ax: Tutorial (initial copy)

Summary:
N428895 is the ANP version of this tutorial. This will likely evolve more before making it to the website, but checking in the initial version.

There will be two modular BoTorchModel tutorials: usage (this one) and customizing (another one in the works, N428917).

Reviewed By: sdsingh

Differential Revision: D25730856

fbshipit-source-id: 7e307c47463ee1e1d65f2e9c48aee5fadedf856c