- Default values of cov and inv variables are now 0. Zero-debiasing (as in Adam...

- Default values of cov and inv variables are now 0.  Zero-debiasing (as in Adam) is used for the cov matrices.  Note this this requires that cov variables, then inv variables, are all updated before the first training update is made.  All examples have been modified to do this. NOTE: you *may* have to increase the damping value you use at the start of optimization after this change (or throughout, if you are using a constant value).
- Changed the initial default approximation used for generic registrations to "diagonal"
- Convenience properties for ops and thunks have all been removed, along with "make_ops_and_vars". User should only interface with "make_vars_and_create_op_thunks" (or maybe "create_ops_and_vars_thunks").

PiperOrigin-RevId: 194461623