randrange() error when running pregenerate_training_data.py code in lm_finetuning
Hi,

I am trying to run pregenerate_training_data.py code in lm_finetuning using a text file which has two documents ( each document has around 200 sentences )
I ran into this error:

```
Better speed can be achieved with apex installed from https://www.github.com/nvidia/apex.
Loading Dataset: 399 lines [00:00, 2214.07 lines/s]
Epoch:   0%|                                                                                                                                                                  | 0/3 [00:00<?, ?it/s]
Traceback (most recent call last):                                                                                                                                            | 0/1 [00:00<?, ?it/s]
  File "pregenerate_training_data.py", line 292, in <module>
    main()
  File "pregenerate_training_data.py", line 277, in main
    vocab_list=vocab_list)
  File "pregenerate_training_data.py", line 187, in create_instances_from_document
    random_document = doc_database.sample_doc(current_idx=doc_idx, sentence_weighted=True)
  File "pregenerate_training_data.py", line 52, in sample_doc
    sentence_index = randint(rand_start, rand_end-1) % self.cumsum_max
  File "/home/cloud/anaconda3/lib/python3.6/random.py", line 221, in randint
    return self.randrange(a, b+1)
  File "/home/cloud/anaconda3/lib/python3.6/random.py", line 199, in randrange
    raise ValueError("empty range for randrange() (%d,%d, %d)" % (istart, istop, width))
ValueError: empty range for randrange() (198,198, 0)
```

command line prompt looks like

`python pregenerate_training_data.py --train_corpus=./ack_belief_training_testing/ack_belief_all_categories_data2.txt --bert_model=bert-base-uncased --do_lower_case --output_dir=./ack_belief_training_testing/pytorch_gen_data/ack_belief_all_categories_data2_train_data_3epochs/ --epochs_to_generate=3`

What can be the issue? 
Can somebody help me through this?

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