Checkpointable: Make Templates Checkpointable

Uses a variable_creator_scope to hook all variable creation within the
Template. For variables without a more deeply nested Template parent, it adds a
dependency directly. For variables with a Template parent, it adds a dependency
on the sub-Template instead.

The variable scope prefix for the Template itself is stripped. However, any
variable_scopes inside the Template (such as those for Layers) will be included
in the dependency names. So within Templates we essentially have name-based
saving (with the exception of dependencies between Templates themselves, which
use the object-based dependency mechanism). This isn't ideal, but will hopefully
allow migration toward object oriented dependencies more smoothly.

Throws an error on object-based save() if the dependencies don't match
between Checkpointable and .variables.

Includes a semi-related usability fix for the Checkpoint utility; mostly in
unit tests, restore() is not called before save(), which when graph building
was leading to the save counter not being initialized. Fixes that.

PiperOrigin-RevId: 187560911