code: def test_num_workers(self):
      dataset = MyDataset()
      dl = JittorDataLoader(dataset, batch_size=4, num_workers=2)
      for idx, batch in enumerate(dl):
          assert batch.shape == [4, 3, 4]
      for idx, batch in enumerate(dl):
          assert batch.shape == [4, 3, 4]
  
file_path: fastNLP/tests/core/dataloaders/jittor_dataloader/test_fdl.py
imports: import pytest
  import numpy
  from fastNLP.envs import _module_available
  from fastNLP.core.dataloaders.jittor_dataloader import JittorDataLoader, prepare_jittor_dataloader
  from fastNLP.core.dataset import DataSet
  from fastNLP.core.collators import Collator
  from fastNLP.io.data_bundle import DataBundle
  from fastNLP.envs.imports import _NEED_IMPORT_JITTOR
label_desc: Use image node_id 4 for calling the TestJittor obj's underlying member method code with example usage: obj.test_num_workers() without return types