fix custom botorch model in ax tutorial (#2618)

Summary:
Pull Request resolved: #2618

Update to use SurrogateSpec+ ModelConfigs. Passing arguments (not kwargs) broke with recent changes in Ax

Reviewed By: Balandat

Differential Revision: D65605144

fbshipit-source-id: 5ce1c99d324171a4088b69cfc37cbb8e2f9bfd62

Author: sdaulton