Only initializes the variables of a TensorFlow session that were not
  already initialized.
  :param sess: the TensorFlow session
  :return:

def initialize_uninitialized_global_variables(sess):
  """
  Only initializes the variables of a TensorFlow session that were not
  already initialized.
  :param sess: the TensorFlow session
  :return:
  """
  # List all global variables
  global_vars = tf.global_variables()

  # Find initialized status for all variables
  is_var_init = [tf.is_variable_initialized(var) for var in global_vars]
  is_initialized = sess.run(is_var_init)

  # List all variables that were not initialized previously
  not_initialized_vars = [var for (var, init) in
                          zip(global_vars, is_initialized) if not init]

  # Initialize all uninitialized variables found, if any
  if len(not_initialized_vars):
    sess.run(tf.variables_initializer(not_initialized_vars))