class_docstr: Pairwise Loss for Reward Model
  Details: https://arxiv.org/abs/2203.02155
code: def forward(
      self, chosen_reward: torch.Tensor, reject_reward: torch.Tensor
  ) -> torch.Tensor:
      probs = torch.sigmoid(chosen_reward - reject_reward)
      log_probs = torch.log(probs)
      loss = -log_probs.mean()
      return loss
  
imports: from typing import Optional
  import torch
  import torch.nn
  from .utils import masked_mean
label_desc: Use image node_id 1 for calling the LogSigLoss obj's underlying member method code with example usage: obj.forward(chosen_reward, reject_reward) and returns: loss