'add_samplewise_zero_center.
Zero center every sample with specified mean. If not specified,
the mean is evaluated over all samples.
Arguments:
mean: `float` (optional). Provides a custom mean. If none
provided, it will be automatically caluclated based on
the training dataset. Default: None.
per_channel: `bool`. If True, compute mean per color channel.
Returns:
Nothing.'

def add_featurewise_zero_center(self, mean=None, per_channel=False):

    self.global_mean.is_required = True
    self.global_mean.value = mean
    if per_channel:
        self.global_mean_pc = True
    self.methods.append(self._featurewise_zero_center)
    self.args.append(None)
