I'm not using this, but I saw by accident while browsing the code that keras.backend.openvino.core.custom_gradient() is a function that defines nested functions __init__() and __call__(), but doesn't do anything (in particular, it seems to return None):

I think line 598 should be

class custom_gradient:

Introduced by 5c401a9 in #19727. It was a function throwing a NotImplementedError before, but should have been changed to a class.