class_docstr: An operator that combines output blocks from
  two or more input operators into a single output.
code: def _add_input_inner(self, refs: RefBundle, input_index: int) -> None:
      assert not self.completed()
      assert (
          0 <= input_index <= len(self._input_dependencies)
      ), input_index
  
      if not self._preserve_order:
          self._output_buffer.append(refs)
      else:
          if input_index == self._input_idx_to_output:
              self._output_buffer.append(refs)
          else:
              self._input_buffers[input_index].append(refs)
  
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 4 for calling the UnionOperator obj's underlying member method code with example usage: obj._add_input_inner(refs, input_index) without return types