* dropping support for special AD handling for hcb.id_tap and id_print.
From now on, only the primals are tapped. The old behavior can be
obtained (for a limited time) by setting the JAX_HOST_CALLBACK_AD_TRANSFORMS
environment variale, or the --flax_host_callback_ad_transforms flag.
Additionally, added documentation for how to implement the old behavior
using JAX custom AD APIs.
This allows us to make some significant cleanup in the internals.