keras/src/callbacks/callback.py/Callback/on_epoch_begin
class Callback:    def on_epoch_begin(self, epoch, logs=None):
        """Called at the start of an epoch.

        Subclasses should override for any actions to run. This function should
        only be called during TRAIN mode.

        Args:
            epoch: Integer, index of epoch.
            logs: Dict. Currently no data is passed to this argument for this
              method but that may change in the future.
        """