Switch deep-kernel to use testbed losses and l2 weight decay. Other refactoring and cleanup.
Switch deep-kernel to use testbed losses and l2 weight decay. Other refactoring and cleanup.

PiperOrigin-RevId: 401069763
Change-Id: Ib19848207ef2f2cf06ea53667f5d8be849d263dc

deepmind/neural_testbed,deepmind/neural_testbed