code: def soft_target_update(self):
      
      for target_param, local_param in zip(self.target.parameters(), self.local.parameters()):
        target_param.data.copy_(self.tau * local_param.data + (1.0 - self.tau) * target_param.data)
docs: Soft update model parameters:
  
      .. math::
        \\theta_target = \\tau \\times \\theta_local + (1 - \\tau) \\times \\theta_target ,
        with \\tau \\ll 1
  
      See https://arxiv.org/pdf/1509.02971.pdf