code: def constant_schedule(pmf):
      """Returns a schedule tuple for constant sampling distribution.
  
      Args:
        pmf: An array of shape [N] of probabilities. The sampling distribution to
          use throughout training. Probabilities must sum to one.
  
      Returns:
        A schedule tuple, see encode_schedule for details.
      """
      return ("step", (0,), (tuplize(pmf),))
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import functools
  import os
  import numpy
  from tensor2tensor.data_generators import problem
  from tensor2tensor.data_generators import text_problems
  import tensorflow.compat.v1
label_desc: Use image node_id 6 for calling a global function with example usage: constant_schedule(pmf) and returns: str,  int,  tuplize