How do I delete photos only from my iPad but not from iCloud
You don’t need to worry about deleting your photos. If you download apps and things, photos will dynamically remove pictures to free up space. It knows it is backed up in iCloud, so it doesn’t worry about keeping your photos saved on device.

edit: If you absolutely  must have them gone, i'd suggest turning off iCloud photo library for the device in question. go to setting  and tap on your name, click on iCloud, then photos. Turn it off and tap remove from this device. remember you won't get any further photos synced to that device until you turn it back on.