} else {
    std::array<Gradient*, 2> to_aggregate;
    to_aggregate[0] = tangent;
    to_aggregate[1] = existing->second;
    // AggregateGradients steals a reference to each of its arguments. We
    // MarkAsResult on `tangent` above so we don't steal a reference to it.
    existing->second = vspace_.AggregateGradients(to_aggregate);
  }
}

  // and not for correctness. The only downside of keeping this 1 seems to be
  // that the gradient accumulation is unbounded and we will never
  // aggressively aggregate accumulated gradients to recover memory.
  // Revisit and fix.
  return 1;
}

// Consumes references to the tensors in the gradient_tensors list and returns
// a tensor with the result.