Dolby vision HDR works on the iPad Pro 10.5 and the 2018 iPad Pros...but its not true HDR and the contrast and brightness is limited. If the screens were OLED, it would be much better but it tries to simulate mobile HDR which isn't quite good. Also, I don't think any of the iPads are true 10 bit displays.....
Try Netflix for the Dolby Vision HDR movies or itunes movies.....