Move optimized function graph proto into graph construction to avoid copies.
Move optimized function graph proto into graph construction to avoid copies.

Previously we were attempting to `std::move()` a `const GraphDef&` which led to the copying overload of `ConvertGraphDefToGraph(..., const GraphDef&, ...)` being called. After this change we properly call the `ConvertGraphDefToGraph(..., GraphDef&&, ...)` overload, which avoids the memory overheads of copying each `NodeDef` in the graph.
