Introduce a Graph Verifier interface in Grappler.

This can be used to implement specific graph verifiers that can then be plugged into the overall grappler workflow or run standalone.
Some examples of specific verifiers are:
 1. A general structural verifier that verifies that the specified graph has
   a valid structure that meets the specification of what it means to be
      a valid TF graph.
 2. A backend specific verifier that verifies that the specified graph,
     generated after a grappler transformation to convert the input TF graph
     to a corresponding backend graph, is a valid graph in the specification
     of the backend.

PiperOrigin-RevId: 229832249