dev(narugo): register weight as buffer

    def __init__(self, weight=None, gamma=2., reduction='mean'):

        nn.Module.__init__(self)

        self.weight = torch.as_tensor(weight).float() if weight is not None else weight

        weight = torch.as_tensor(weight).float() if weight is not None else weight

        self.register_buffer('weight', weight)

        self.gamma = gamma

        self.reduction = reduction