code: def dmol_neg_log_perplexity(predictions, labels, weights_fn=None):
      """Average log-perplexity excluding padding 0s. No smoothing."""
      del weights_fn  # Unused
      num, den = common_layers.dml_loss(
          predictions, labels, reduce_sum=False
      )
      return (-num, den)
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import numpy
  import six
  from tensor2tensor.layers import common_layers
  from tensor2tensor.layers import modalities
  from tensor2tensor.utils import bleu_hook
  from tensor2tensor.utils import contrib
  from tensor2tensor.utils import rouge
  from tensor2tensor.utils import sari_hook
  import tensorflow.compat.v1
  from tensorflow.python.util import tf_inspect
label_desc: Use image node_id 17 for calling a global function with example usage: dmol_neg_log_perplexity(predictions, labels, weights_fn) and returns: unknown,  den