class_docstr: 通过该Callback实现训练过程中loss的输出
code: def on_train_epoch_begin(self, trainer):
      self.epoch_start = time.time()
  
imports: import os
  import sys
  import time
  from datasets import load_dataset
  from datasets import DatasetDict
  import jittor
  from jittor import nn, Module
  from jittor.dataset import Dataset
  from fastNLP.core.controllers.trainer import Trainer
  from fastNLP.core.metrics.accuracy import Accuracy
  from fastNLP.core.vocabulary import Vocabulary
  from fastNLP.core.callbacks.progress_callback import RichCallback
  from fastNLP.core.callbacks.callback import Callback
  from fastNLP.core.dataloaders.jittor_dataloader.fdl import JittorDataLoader
label_desc: Use image node_id 4 for calling the PrintWhileTrainingCallBack obj's underlying member method code with example usage: obj.on_train_epoch_begin(trainer) without return types