Add powerSign optimizer to deepchem

      tfopt = opt._create_optimizer(global_step)

      assert isinstance(tfopt, tf.train.GradientDescentOptimizer)

  def test_power_sign(self):

    """Test creating a power sign optimizer."""

    opt = optimizers.PowerSignOptimizer(learning_rate=0.01)

    with self.test_session() as sess:

      global_step = tf.Variable(0)

      tfopt = opt._create_optimizer(global_step)

      assert isinstance(tfopt, tf.contrib.opt.PowerSignOptimizer)

  def test_exponential_decay(self):

    """Test creating an optimizer with an exponentially decaying learning rate."""

    rate = optimizers.ExponentialDecay(