Bad F1 Score for run_squad.py on SQuAD2.0
## ❓ Questions & Help

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When I run the run_squad.py for SQuAD2.0 like this

python3 run_squad.py \
  --model_type bert \
  --model_name_or_path bert-base-cased \
  --do_train \
  --do_eval \
  --train_file /share/nas165/Wendy/transformers/examples/tests_samples/SQUAD/train-v2.0.json \
  --predict_file /share/nas165/Wendy/transformers/examples/tests_samples/SQUAD/dev-v2.0.json \
  --per_gpu_train_batch_size 4 \
  --learning_rate 4e-5 \
  --num_train_epochs 2.0 \
  --max_seq_length 384 \
  --doc_stride 128 \
  --output_dir /share/nas165/Wendy/transformers/examples/SQuAD2.0_debug_bert/
  --version_2_with_negative=True \
  --null_score_diff_threshold=-1.967471694946289

It runs very fast and the f1 score only 7.9% What is wrong with it
The log message like this
<img width="960" alt="擷取" src="https://user-images.githubusercontent.com/32416416/71322341-939daa80-2501-11ea-9313-e179d1760b99.PNG">
Thanks a lot for your help.By the way I clone it today so it is the new version
