message: Create a tf.constant for the weak_learner_type that's shared across all the
  handlers to avoid duplicate constant creation.
diff: @@ -672,6 +672,8 @@ class GradientBoostedDecisionTreeModel(object):
  self._learner_config.constraints.min_node_weight, dtypes.float32)
  loss_uses_sum_reduction = self._loss_reduction == losses.Reduction.SUM
  loss_uses_sum_reduction = constant_op.constant(loss_uses_sum_reduction)
  + weak_learner_type = constant_op.constant(
  + self._learner_config.weak_learner_type)
  epsilon = 0.01
  num_quantiles = 100
  strategy_tensor = constant_op.constant(strategy)
  @@ -696,6 +698,7 @@ class GradientBoostedDecisionTreeModel(object):
  multiclass_strategy=strategy_tensor,
  init_stamp_token=init_stamp_token,
  loss_uses_sum_reduction=loss_uses_sum_reduction,
  + weak_learner_type=weak_learner_type,
  ))
  fc_name_idx += 1
  