Use BOTORCH_MODULAR in tutorials with Ax (#1105)

Summary:
Pull Request resolved: #1105

Update the Ax-related tutorials in BoTorch to use the BOTORCH_MODULAR interface.

For starters, this updates custom BoTorch model tutorial. I will update others based on feedback on the style.

Reviewed By: Balandat

Differential Revision: D34646421

fbshipit-source-id: 9d8ca1c53f4bb29b76db242d198c44ab56092f20

Author: saitcakmak