class_docstr: An operator that combines output blocks from
  two or more input operators into a single output.
code: def start(self, options: ExecutionOptions):
      # Whether to preserve the order of the input data (both the
      # order of the input operators and the order of the blocks within).
      self._preserve_order = options.preserve_order
      super().start(options)
  
file_path: ray/python/ray/data/_internal/execution/operators/union_operator.py
imports: from typing import List
  from ray.data._internal.execution.interfaces import ExecutionOptions, PhysicalOperator, RefBundle
  from ray.data._internal.execution.operators.base_physical_operator import NAryOperator
  from ray.data._internal.stats import StatsDict
label_desc: Use image node_id 2 for calling the UnionOperator obj's underlying member method code with example usage: obj.start(options) without return types