how to do the pre training the model form scratch?
for example, use sample_text.txt
```
python3 examples/lm_finetuning/simple_lm_finetuning.py 
--train_corpus sample_text.txt 
--bert_model bert-base-uncased 
--do_lower_case 
--output_dir finetuned_lm/
```
In addition, you can refer to #385 <|||||>Yes let's keep a single issue on this. Closing in favor of #385.