Train a keras model on a worker and send asynchronous updates
        to parameter server

def train(self, data_iterator):
        """Train a keras model on a worker and send asynchronous updates
        to parameter server
        """
        feature_iterator, label_iterator = tee(data_iterator, 2)
        x_train = np.asarray([x for x, y in feature_iterator])
        y_train = np.asarray([y for x, y in label_iterator])

        if x_train.size == 0:
            return

        optimizer = get_optimizer(self.master_optimizer)
        self.model = model_from_yaml(self.yaml, self.custom_objects)
        self.model.compile(optimizer=optimizer,
                           loss=self.master_loss, metrics=self.master_metrics)
        self.model.set_weights(self.parameters.value)

        epochs = self.train_config['epochs']
        batch_size = self.train_config.get('batch_size')
        nb_train_sample = x_train.shape[0]
        nb_batch = int(np.ceil(nb_train_sample / float(batch_size)))
        index_array = np.arange(nb_train_sample)
        batches = [
            (i * batch_size, min(nb_train_sample, (i + 1) * batch_size))
            for i in range(0, nb_batch)
        ]

        if self.frequency == 'epoch':
            for epoch in range(epochs):
                weights_before_training = self.client.get_parameters()
                self.model.set_weights(weights_before_training)
                self.train_config['epochs'] = 1
                if x_train.shape[0] > batch_size:
                    self.model.fit(x_train, y_train, **self.train_config)
                self.train_config['epochs'] = epochs
                weights_after_training = self.model.get_weights()
                deltas = subtract_params(
                    weights_before_training, weights_after_training)
                self.client.update_parameters(deltas)
        elif self.frequency == 'batch':
            for epoch in range(epochs):
                if x_train.shape[0] > batch_size:
                    for (batch_start, batch_end) in batches:
                        weights_before_training = self.client.get_parameters()
                        self.model.set_weights(weights_before_training)
                        batch_ids = index_array[batch_start:batch_end]
                        x = slice_arrays(x_train, batch_ids)
                        y = slice_arrays(y_train, batch_ids)
                        self.model.train_on_batch(x, y)
                        weights_after_training = self.model.get_weights()
                        deltas = subtract_params(
                            weights_before_training, weights_after_training)
                        self.client.update_parameters(deltas)
        else:
            raise ValueError(
                'frequency parameter can be `epoch` or `batch, got {}'.format(self.frequency))
        yield []