Cleanup and consolidation: Add a GraphRunner::Run function runs a

graph.  This is similar to what is already being done in
ConstantFolding, but allows us to feed inputs too.  This function
replaces some of the code in ConstantFolding, and will also be used
for the purposes of running constant graphs for shape inference (I
have another CL that will use this, once checked in.  The other CL was
using DirectSession as a proof of concept, but we need something
simpler to avoid layering violations).

I have documented the class / function as not super general and only
really useful for constant evaluations.
Change: 133331175