Zero-mean, unit-variance normalization by default

def _preprocess(self, data,  train):
        """Zero-mean, unit-variance normalization by default"""
        if train:
            inputs, labels = data
            self.data_mean = inputs.mean(axis=0)
            self.data_std = inputs.std(axis=0)
            self.labels_mean = labels.mean(axis=0)
            self.labels_std = labels.std(axis=0)
            return ((inputs-self.data_mean)/self.data_std, (labels-self.labels_mean)/self.labels_std)
        else:
            return (data-self.data_mean)/self.data_std