Build the GRU cell.

def build(self, x, h, mask=None):
        '''
        Build the GRU cell.
        '''
        xw = tf.split(tf.matmul(x, self.w_matrix) + self.bias, 3, 1)
        hu = tf.split(tf.matmul(h, self.U), 3, 1)
        r = tf.sigmoid(xw[0] + hu[0])
        z = tf.sigmoid(xw[1] + hu[1])
        h1 = tf.tanh(xw[2] + r * hu[2])
        next_h = h1 * (1 - z) + h * z
        if mask is not None:
            next_h = next_h * mask + h * (1 - mask)
        return next_h