'Call all registered callbacks\' on_epoch_begin functions.
Arguments:
epoch (int): index of epoch that is beginning'

def on_epoch_begin(self, epoch):

    for c in self.callbacks:
        if c.should_fire(self.callback_data, self.model(), epoch, c.epoch_freq):
            c.on_epoch_begin(self.callback_data, self.model(), epoch)
