Add a utility to visualize object-based checkpoints

Useful for generating a warm fuzzy feeling that everything you think should be saved was saved, and for explaining what object-based checkpointing is. (Also useful on the former front will be a planned "assert that all of this Graph's trainable variables are accessible from object X" function.)

Somewhat hacky since it generates strings rather than using the pydot bindings (and so works without a pydot dependency).

PiperOrigin-RevId: 193708003