text: Title: run_pretraining.py ignores undefined flags.
  Body: Hi, I'm trying to make bert model.
  
  I found that **`run_pretraining.py` ignores undefined flags**.
  
  for example, If I do like this,
  ```
  python3 bert/run_pretraining.py \
  --input_file=gs://input_dir/*.tfrecord \
  --output_dir=gs://output_dir/ \
  --do_train=True \
  --do_eval=True \
  --bert_config_file=./bert_config.json \
  --train_batch_size=1024 \
  --max_seq_length=128 \
  --max_predictions_per_seq=20 \
  --num_train_steps=1000000 \
  --num_warmup_steps=10000 \
  --learning_rate=5e-5 \
  --save_checkpoints_steps=10000 \
  --init_checkpoints=340000 \
  --use_tpu=True \
  --tpu_name=tpu2 \
  --tpu_zone=us-central1-f \
  --fake=undifined_flags
  --gcp_project=my_project \
  --num_tpu_cores=8
  ```
  
  **I included an undefined arg** **`--fake=undifined_flags`**
  I think It should throw out an error, but It doesn't.
  It train well. (Maybe) There is no problem in progress.
  
  TPU was connected normally, and ckpt was made normally too.
  And fine tuning result of ckpt is not bad. 
  
  Why it doesn't throw out error? Why it works?
  
  
  
  