message: Delegate _aggregate_gradients in LossScaleOptimizer.
  This is needed for
diff: @@ -333,6 +333,9 @@ class LossScaleOptimizer(optimizer_v2.OptimizerV2):
  def set_weights(self, weights):
  return self._optimizer.set_weights(weights)
  + def _aggregate_gradients(self, grads_and_vars):
  + return self._optimizer._aggregate_gradients(grads_and_vars) # pylint: disable=protected-access
  +
  # For the most part, we only expose methods in the base OptimizerV2, not
  # individual subclasses like Adam. However, although "learning_rate" and "lr"
  # properties are not part of the base OptimizerV2 class, they are part of most
  