class_docstr: WordVarLenTensorGenerator generates string tf.SparseTensor with string
  length similar to a word.
code: def generate(self):
      # Use Poisson distribution to sample the length of byte strings.
      # The avg_length equals to the lambda in Poisson distribution.
      shape = self._get_shape()
      idxs = []
      self._get_idx(0, shape, [0] * len(shape), idxs)
      lengths = np.random.poisson(self._avg_length, size=len(idxs))
  
      to_string = lambda length: np.random.bytes(length)
      vfunc = np.vectorize(to_string)
      vals = vfunc(lengths)
      return tf.SparseTensor(
          indices=idxs, values=vals, dense_shape=shape
      )
  
imports: import enum
  import hashlib
  import numpy
  import tensorflow
  from tests.test_atds_avro.utils.generator.generator_base import Generator
  from tests.test_atds_avro.utils.hash_util import int_to_bytes
label_desc: Use image node_id 2 for calling the WordVarLenTensorGenerator obj's underlying member method code with example usage: obj.generate() and returns: tf