Classes needed during training.

def _init_training(self):
        # pylint: disable=redefined-variable-type
        """Classes needed during training."""
        if self.check:
            self.backprop = CheckedBackprop(self.network, self.problem.cost)
        else:
            self.backprop = BatchBackprop(self.network, self.problem.cost)
        self.momentum = Momentum()
        self.decent = GradientDecent()
        self.decay = WeightDecay()
        self.tying = WeightTying(*self.problem.weight_tying)
        self.weights = self.tying(self.weights)