code: def initialize(self):
      self.global_step = tf.Variable(
          tf.zeros([], dtype=dtypes.int32),
          dtype=dtypes.int32,
          name="global_step",
      )
  
      def training_step_fn():
          return self.global_step
  
      self.training_step_fn = training_step_fn
  
      compat.initialize_variables(self)
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from absl.testing import parameterized
  import numpy
  import tensorflow
  from tensorflow_model_optimization.python.core.keras import compat
  from tensorflow_model_optimization.python.core.sparsity.keras import pruning_impl
  from tensorflow_model_optimization.python.core.sparsity.keras import pruning_schedule
label_desc: Use image node_id 2 for calling the PruningTest obj's underlying member method code with example usage: obj.initialize() and returns: self