code: def __init__(self, vocab, preprocessing=None, postprocessing=None):
      self.vocab = vocab
      self.preprocessing = preprocessing
      self.postprocessing = postprocessing
  
file_path: dgl/examples/pytorch/transformer/dataset/fields.py
label_desc: Use image node_id 1 to create a new Field object  with example: obj = Field(vocab, preprocessing, postprocessing)