This CL switches the custom-registered-gradient of nested Functions from using...

This CL switches the custom-registered-gradient of nested Functions from using the C++ SymGrad to the correct forward & backward passes. This allows gradients of defuns nested > 2 levels deep to work correctly.

This CL also adds a number of unit tests to make sure various nestings of gradient & defuns work. (Note: unit tests that combine multiple levels of gradient tapes & multiple levels of defun still need to be added).

PiperOrigin-RevId: 216939066