'Return the optimizer used in the task.
Used by some of the tensorpack :class:`Trainer` which assume single optimizer.
You should use :class:`ModelDescBase` if you use a custom trainer with more than one optimizers.
Users of :class:`ModelDesc` will need to implement `_get_optimizer()`,
which will only be called once per each model.
Returns:
a :class:`tf.train.Optimizer` instance.'

@memoized
def get_optimizer(self):

    return self._get_optimizer()
