keras/src/callbacks/tensorboard.py/TensorBoard/on_epoch_begin
class TensorBoard:    def on_epoch_begin(self, epoch, logs=None):
        # Keeps track of epoch for profiling.
        if self.write_steps_per_second:
            self._previous_epoch_iterations = ops.convert_to_tensor(
                self.model.optimizer.iterations, "float32"
            )
            self._epoch_start_time = time.time()