docstr: generate fake sequence indexes in variant length
docstr_tok: ["generate","fake","sequence","indexes","in","variant","length"]
code: def gen_var_seq(self, max_len, vocab_size):
      """generate fake sequence indexes in variant length"""
      length = randrange(3, max_len)  # at least 3 words in a seq
      return self.gen_seq(length, vocab_size)
  
file_path: fastNLP/tests/models/torch/model_runner.py
imports: from fastNLP.envs.imports import _NEED_IMPORT_TORCH
  from fastNLP import Trainer, Evaluator, DataSet, Callback
  from fastNLP import Accuracy
  from random import randrange
  from fastNLP import TorchDataLoader
label_desc: Use image node_id 2 for calling the ModelRunner obj's underlying member method code with example usage: obj.gen_var_seq(max_len, vocab_size) and returns: self