code: def forward(self, sign, score):
      return -torch.mean(torch.log(torch.sigmoid(sign * score)))
  
imports: import random
  import numpy
  import torch
  import torch.nn
  from recbole.model.abstract_recommender import GeneralRecommender
  from recbole.model.init import xavier_normal_initialization
  from recbole.utils import InputType
label_desc: Use image node_id 2 for calling the NegSamplingLoss obj's underlying member method code with example usage: obj.forward(sign, score) and returns: unknown