class_docstr: 基础的 Jittor 分类模型
class_docstr_tok: ["\u57fa\u7840\u7684","Jittor","\u5206\u7c7b\u6a21\u578b"]
code: def train_step(self, x, y):
      x = self(x)
      return {"loss": self.loss_fn(x, y)}
  
imports: import pytest
  from fastNLP.core.callbacks import callback
  from fastNLP.core.controllers.trainer import Trainer
  from fastNLP.core.controllers.trainer import Evaluator
  from fastNLP.core.metrics.accuracy import Accuracy
  from fastNLP.core.callbacks.progress_callback import RichCallback
  from fastNLP.core.dataloaders.jittor_dataloader.fdl import JittorDataLoader
  from fastNLP.envs.imports import _NEED_IMPORT_JITTOR
label_desc: Use image node_id 3 for calling the JittorNormalModel_Classification obj's underlying member method code with example usage: obj.train_step(x, y) and returns: dict