// propagates gradients `dst_grad` backwards along the edge from `src`
  // to `dst` in the graph. This will add `dst_grad` to the list of pending
  // gradients for the node associated with `src`.
  Status BackpropAlongEdge(const Output& dst_grad, const Output& src);

  // Adds a node to the graph (returned in `grad`) that sums the in-bound
  // gradients to `src` (if there are more than one).