def partial_fit(self, X):
        """Train model based on mini-batch of input data.
        
        Return cost of mini-batch.
        """
        opt, cost = self.sess.run((self.optimizer, self.cost), 
                                  feed_dict={self.x: X})
        return cost
Train model based on mini-batch of input data.
        
        Return cost of mini-batch.