How to sync Faces between devices in iCloud Photo Library?
Unfortunately faces are not synched across devices using iCloud Photo Library.

There is evidence that Faces data is stored in Apple servers somewhere since you can ask Siri to show you pictures of a specific person from a different device that uses the same iCloud photo library. There is a reddit thread on this topic here:  https://www.reddit.com/r/apple/comments/3ggvn3/icloud_photo_library_and_faces/.

My guess is that Apple is still grappling with how to create a consolidated view of your contacts across devices and photos. 

Please request that Apple prioritize fully implementing face syncing across devices:  http://www.apple.com/feedback/photos.html.