message: Skip calling back into python if only 1 gradient to aggregate
diff: @@ -520,7 +520,12 @@ Status GradientTape<Gradient, BackwardFunction>::ComputeGradient(
  }
  } else {
  any_gradient_nonzero = true;
  - auto new_gradients = vspace.AggregateGradients(grad_it->second);
  + Gradient* new_gradients = nullptr;
  + if (grad_it->second.size() == 1) {
  + new_gradients = grad_it->second.at(0);
  + } else {
  + new_gradients = vspace.AggregateGradients(grad_it->second);
  + }
  if (sources_set.find(grad_it->first) == sources_set.end()) {
  gradients.erase(grad_it);
  } else {
  