In a large bowl, whisk milk and pudding mix for 2 minutes. Transfer to a greased 5-qt. slow cooker. Prepare cake mix batter according to package directions, folding cherries into batter. Pour into slow cooker.
Cook, covered, on low until edges of cake are golden brown, about 4 hours.
Remove slow-cooker insert; sprinkle cake with baking chips and chocolate chips. Let cake stand, uncovered, 10 minutes. Sprinkle with pistachios before serving.