Among the students who took the course Machine Learning Theory, how many of them are undergraduates?
UG is an abbreviated name of undergraduate student in which type = 'UG';
SELECT COUNT(T1.student_id) FROM student AS T1 INNER JOIN registration AS T2 ON T1.student_id = T2.student_id INNER JOIN course AS T3 ON T2.course_id = T3.course_id WHERE T3.name = 'Machine Learning Theory' AND T1.type = 'UG'