docstr: Loss function for supervised problems.
code: def criterion(self):
      """Loss function for supervised problems."""
      return self._criterion(self.mb_output, self.mb_y)
  
file_path: avalanche/avalanche/training/templates/problem_type/supervised_problem.py
imports: from avalanche.models import avalanche_forward
  from avalanche.training.templates.strategy_mixin_protocol import SupervisedStrategyProtocol
label_desc: Use image node_id 4 for calling the SupervisedProblem obj's underlying member method code with example usage: obj.criterion() and returns: self