run_squad.py crashes during do_eval
## 🐛 Bug

When running run_squad.py as provided in the README once training is complete the prediction/evaluation component of the script crashes.

No prediction files are written are written.

Model I am using (Bert, XLNet....): Bert

Language I am using the model on (English, Chinese....): English

The problem arise when using:
* [ ] the official example scripts: (give details) example squad fine tuning

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name) official squad dev set 1.1
* [ ] my own task or dataset: (give details) the checkpoint was made using a custom training dataset in squad format, but it appears to be an eval bug

## To Reproduce

Steps to reproduce the behavior:

1. finish training as specified in README
2. I ran with this command
CUDA_VISIBLE_DEVICES=10,11,12,13,14,15 python -m torch.distributed.launch --nproc_per_node=6 ./examples/run_squad.py     --model_type bert     --model_name_or_path bert-large-uncased-whole-word-masking     --do_train     --do_eval     --do_lower_case     --train_file /data/data/SQUAD/train-v1.1json     --predict_file /data/data/SQUAD/dev-v1.1.json     --learning_rate 3e-5     --num_train_epochs 2     --max_seq_length 384     --doc_stride 128     --output_dir models/wwm_uncased_finetuned_squad_supp/     --per_gpu_eval_batch_size=6       --per_gpu_train_batch_size=6 --save_steps 500

I also ran with just do_eval using the same model and it produced the same error

<!-- If you have a code sample, error messages, stack traces, please provide it here as well. -->

11/26/2019 18:29:01 - INFO - __main__ -   Saving features into cached file /data/data/SQUAD/cached_dev_bert-large-uncased-whole-word-masking_384
11/26/2019 18:29:20 - INFO - __main__ -   ***** Running evaluation  *****
11/26/2019 18:29:20 - INFO - __main__ -     Num examples = 10833
11/26/2019 18:29:20 - INFO - __main__ -     Batch size = 6
Evaluating: 100%|███████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 301/301 [00:48<00:00,  6.20it/s]
11/26/2019 18:30:09 - INFO - __main__ -     Evaluation done in total 48.868770 secs (0.004511 sec per example)
11/26/2019 18:30:09 - INFO - utils_squad -   Writing predictions to: models/wwm_uncased_finetuned_squad_supp/predictions_.json
11/26/2019 18:30:09 - INFO - utils_squad -   Writing nbest to: models/wwm_uncased_finetuned_squad_supp/nbest_predictions_.json
Traceback (most recent call last):
  File "./examples/run_squad.py", line 573, in <module>
    main()
  File "./examples/run_squad.py", line 562, in main
    result = evaluate(args, model, tokenizer, prefix=global_step)
  File "./examples/run_squad.py", line 284, in evaluate
    args.version_2_with_negative, args.null_score_diff_threshold)
  File "/home/clong/git/transformers/examples/utils_squad.py", line 532, in write_predictions
    result = unique_id_to_result[feature.unique_id]
KeyError: 1000000000
Traceback (most recent call last):
  File "/opt/conda/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/opt/conda/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/opt/conda/lib/python3.6/site-packages/torch/distributed/launch.py", line 253, in <module>
    main()
  File "/opt/conda/lib/python3.6/site-packages/torch/distributed/launch.py", line 249, in main
    cmd=cmd)
subprocess.CalledProcessError: Command '['/opt/conda/bin/python', '-u', './examples/run_squad.py', '--local_rank=5', '--model_type', 'bert', '--model_name_or_path', 'bert-large-uncased-whole-word-masking', '--do_eval', '--do_lower_case', '--train_file', '/data/data/SQUAD/train-v1.1.json', '--predict_file', '/data/data/SQUAD/dev-v1.1.json', '--learning_rate', '3e-5', '--num_train_epochs', '2', '--max_seq_length', '384', '--doc_stride', '128', '--output_dir', 'models/wwm_uncased_finetuned_squad_supp/', '--per_gpu_eval_batch_size=6', '--per_gpu_train_batch_size=6', '--save_steps', '500']' returned non-zero exit status 1.

## Expected behavior

no crashing and predictions written

## Environment

* OS: Ubuntu 18.04 in NVIDIA pytorch container
* Python version: 3.6.9 anaconda
* PyTorch version: 1.3.0 custom nvidia version
* PyTorch Transformers version (or branch): pip install
* Using GPU ? yes
* Distributed of parallel setup ? using 6 of 16 gpu's on system
* Any other relevant information: 


same issue<|||||>I tracked it down further this morning and found the problem, you cannot run do_eval in pytorch distributed mode, do_eval works completely fine when there is no pytorch distributed in the equation.  This should probably result in a change to the README

<|||||>👍Just ran it, I confirm that do_eval runs well without distributed mode<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
