Fix custom_botorch_model_in_ax tutorial (#2641)

Summary:
Updates the tutorial to pass in `input_transform_classes=None` (to disable default transforms), since the custom model does not support transforms.

Pull Request resolved: #2641

Reviewed By: Balandat

Differential Revision: D66668511

Pulled By: saitcakmak

fbshipit-source-id: 15c04ab0d76da15d2891c7a56ce83b3f2f6d5794