class_docstr: 单独实现 train_step 和 evaluate_step；
code: def train_step(self, x, y):
      x = self(x)
      return {"loss": self.loss_fn(x, y)}
  
imports: from fastNLP.envs.imports import _NEED_IMPORT_ONEFLOW
label_desc: Use image node_id 3 for calling the OneflowNormalModel_Classification_1 obj's underlying member method code with example usage: obj.train_step(x, y) and returns: dict