Domain shift is a fundamental problem in machine learning, that has attracted a lot of attention in the natural language processing and vision communities [2, 6, 11,<cite> 13,</cite> 29, 30, 32, 37, 39, 40, 42] . To understand and address this problem, generated by the lack of labeled data in a target domain, researchers have studied the behavior of machine learning methods in cross-domain settings [12,<cite> 13,</cite> 29] and came up with various domain adaptation techniques [6, 11, 28, 39] .