code: def moments_over_bl(x, x_mask):
      """Returns mean and var of x over B and L."""
      mean = reduce_mean_over_bl(x, x_mask)
      var = reduce_mean_over_bl((x - mean) ** 2, x_mask)
      return mean, var
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import copy
  import math
  from tensor2tensor.layers import common_attention
  from tensor2tensor.layers import common_layers
  from tensor2tensor.models.transformer import transformer_decoder_layer
  import tensorflow.compat.v1
  import tensorflow_probability
label_desc: Use image node_id 10 for calling a global function with example usage: moments_over_bl(x, x_mask) and returns: mean,  var