code: def _must_be_fed(op):
      return op.type == "Placeholder"
  
imports: import collections
  import os
  import time
  from tensorflow.python.lib.io import file_io
  from tensorflow.python.ops import op_selector
  from tensorflow.python.platform import gfile
  from tensorflow.python.platform import tf_logging
  from tensorflow.python.saved_model import signature_constants
  from tensorflow.python.saved_model import signature_def_utils
  from tensorflow.python.saved_model import tag_constants
  from tensorflow.python.saved_model import utils
  from tensorflow.python.saved_model.model_utils import export_output
  from tensorflow.python.saved_model.model_utils import mode_keys
  from tensorflow.python.saved_model.model_utils.mode_keys import KerasModeKeys
  from tensorflow.python.util import compat
  from tensorflow.python.util import nest
  from tensorflow.python.util import object_identity
label_desc: Use image node_id 1 for calling a global function with example usage: _must_be_fed(op) and returns: unknown