code: def __len__(self):
      return self.num_of_data
  
file_path: fastNLP/tests/helpers/datasets/oneflow_data.py
imports: from fastNLP.envs.imports import _NEED_IMPORT_ONEFLOW
label_desc: Use image node_id 2 for calling the OneflowNormalDataset obj's underlying member method code with example usage: obj.__len__() and returns: self