'add_featurewise_stdnorm.
Scale each sample by the specified standard deviation. If no std
specified, std is evaluated over all samples data.
Arguments:
std: `float` (optional). Provides a custom standard derivation.
If none provided, it will be automatically caluclated based on
the training dataset. Default: None.
per_channel: `bool`. If True, compute std per color channel.
Returns:
Nothing.'

def add_featurewise_stdnorm(self, std=None, per_channel=False):

    self.global_std.is_required = True
    self.global_std.value = std
    if per_channel:
        self.global_std_pc = True
    self.methods.append(self._featurewise_stdnorm)
    self.args.append(None)
