Update platform APIs to support stateless platform.

[wlngai]

To facilitate time-out and fault tolerance, each benchmark run is initialized in an isolated Unix process. Essentially, the implementation of the Platform interface has become stateless. However, to reuse a loaded graph for multiple benchmark runs, the platform must be able to obtain runtime information regarding the loaded graph.

Instead of assuming that the paths to the loaded graph are deterministic, the load-graph method should return a LoadedGraph object, and the returned LoadedGraph object should be made available in the benchmark run specification.