Creates the Tensorflow 'global_step' variable (see `MonitorContext.global_step_tensor`).
    :param session: Tensorflow session the optimiser is running in
    :return: The variable tensor.

def create_global_step(session: tf.Session) -> tf.Variable:
    """
    Creates the Tensorflow 'global_step' variable (see `MonitorContext.global_step_tensor`).
    :param session: Tensorflow session the optimiser is running in
    :return: The variable tensor.
    """
    global_step_tensor = tf.Variable(0, trainable=False, name="global_step")
    session.run(global_step_tensor.initializer)
    return global_step_tensor