message: Introduce OutputTensor class to internal graph representation
  This is analogous to TF_Output in the C API and Output in the public C++ API.
diff: @@ -259,6 +259,19 @@ class Node {
  TF_DISALLOW_COPY_AND_ASSIGN(Node);
  };
  +// Represents an output of a node, i.e., the `index`-th output of `node`. Note
  +// that a single `OutputTensor` can correspond to multiple `Edge`s if the output
  +// is consumed by multiple destination nodes.
  +struct OutputTensor {
  + Node* node;
  + int index;
  +
  + OutputTensor(Node* n, int i) : node(n), index(i) {}
  + OutputTensor() : node(nullptr), index(0) {}
  +};
  +
  +// TODO(skyewm): add InputTensor if/when necessary
  +
  class Edge {
  public:
  Node* src() const { return src_; }
  