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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def summarize_variables(var_list=None, tag=None):
 """Summarize the variables. Args: var_list: a list of variables; defaults to trainable_variables. tag: name scope of the summary; defaults to training_variables/. """

  if var_list is None:
    var_list = tf.trainable_variables()
  if tag is None:
    tag = "training_variables/"

  name_to_var = {v.name: v for v in var_list}
  for v_name in list(name_to_var):
    v = name_to_var[v_name]
    tf.summary.histogram(tag + v_name, v)