code: def key_by(
      backend: pipeline_backend.PipelineBackend,
      col,
      key_extractor: Callable,
      stage_name: str,
  ):
      return backend.map(
          col,
          lambda el: (key_extractor(el), el),
          f"{stage_name}: key collection by keys from key extractor.",
      )
  
imports: from typing import Type, Dict, Any, Callable, TypeVar, Iterable, List
  from pipeline_dp import pipeline_backend
label_desc: Use image node_id 1 for calling a global function with example usage: key_by(backend, col, key_extractor, stage_name) and returns: backend