code: def size(
      backend: pipeline_backend.PipelineBackend, col, stage_name: str
  ):
      """Returns a one element collection that contains the size of the input
      collection."""
  
      col = backend.map(
          col,
          lambda x: "fake_common_key",
          f"{stage_name}: mapping to the same key",
      )
      col = backend.count_per_element(
          col, f"{stage_name}: counting the number of elements"
      )
      return backend.values(
          col, f"{stage_name}: dropping the fake_common_key"
      )
  
imports: from typing import Type, Dict, Any, Callable, TypeVar, Iterable, List
  from pipeline_dp import pipeline_backend
label_desc: Use image node_id 2 for calling a global function with example usage: size(backend, col, stage_name) and returns: backend