class_docstr: An operator that combines output blocks from
  two or more input operators into a single output.
code: def _get_next_inner(self) -> RefBundle:
      return self._output_buffer.pop(0)
  
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 8 for calling the UnionOperator obj's underlying member method code with example usage: obj._get_next_inner() and returns: self