code: def _get_custom_layer_names(spec):
      """
  
      Returns a list of className fields which appear in the given protobuf spec
  
      Parameters
      ----------
      spec: mlmodel spec
  
      Returns
      -------
  
      set(str) A set of unique className fields of custom layers that appear in the model.
  
      """
      layers = _get_nn_layers(spec)
      layers_out = set()
      for layer in layers:
          if layer.WhichOneof("layer") == "custom":
              layers_out.add(layer.custom.className)
  
      return layers_out
  
imports: import math
  import numpy
  import os
  import six
  import warnings
  import sys
  from coremltools.proto import Model_pb2
  from coremltools.models._deprecation import deprecated
  from .._deps import _HAS_SKLEARN
label_desc: Use image node_id 21 for calling a global function with example usage: _get_custom_layer_names(spec) and returns: layers_out