SummaryGraph: Add adjacency map + numerous changes (#291)

* Adjacency map - map from each op to its predecessor and successor ops
* More robust handling of Gemm nodes scope names (instead of
  increment_instance())
* More consistent handling of ops with the same scope name
* Handle pad + avg pool sequences generated by ONNX trace optimization
  (results in one less op in the graph, hence the changes in tests)
* Minor refactoring in predecessors() and successors() functions