message: Fix comments for Subgraph::execution_plan()
diff: @@ -163,10 +163,10 @@ class Subgraph {
  // Return the number of ops in the model.
  size_t nodes_size() const { return nodes_and_registration_.size(); }
  - // Read only access to list of variable tensors.
  + // Return vector of node indices in the order of execution.
  std::vector<int>& execution_plan() { return execution_plan_; }
  - // Read only access to list of variable tensors.
  + // Return read-only vector of node indices in the order of execution.
  const std::vector<int>& execution_plan() const { return execution_plan_; }
  // Mutable form of tensors (TEMPORARY for refactor).
  