code: def _tf_lognormal(y, mean, logstd, logsqrttwopi):
      return (
          -0.5 * ((y - mean) / tf.exp(logstd)) ** 2
          - logstd
          - logsqrttwopi
      )
  
file_path: magenta/magenta/models/svg_vae/svg_decoder_loss.py
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import numpy
  from tensor2tensor.layers import common_layers
  from tensor2tensor.layers import modalities
  import tensorflow.compat.v1
  from tensorflow.compat.v1 import estimator
label_desc: Use image node_id 1 for calling a global function with example usage: _tf_lognormal(y, mean, logstd, logsqrttwopi) and returns: unknown