how do I turn off "find my iPad" when the device is broken and can't be turned on?

You can turn-off Find My iPad (the Activation Lock) from iCloud. Just sign-in from a web browser:

https://icloud.apple.com

Procedures detailed here:

Remove a device from Find My iPhone on iCloud.com - Apple Support

How do I turn off my apple ipad from an android phone when it says my browser is not compatible.