class_docstr: Base class for Diet variable optimizers.
code: def global_step(self):
      return self._global_step
  
imports: from collections import defaultdict
  import copy
  import math
  from tensor2tensor.layers import common_layers
  from tensor2tensor.utils import hparam
  import tensorflow.compat.v1
label_desc: Use image node_id 3 for calling the DietVariableOptimizer obj's underlying member method code with example usage: obj.global_step() and returns: self