Unexpectedly preprocess when multi-gpu using
When run example/run_squad with more than one gpu, the preprocessor cannot work as expected. For example, the unique_id will not be a serial numbers, then keyerror occurs when writing the result to json file.

https://github.com/huggingface/pytorch-transformers/blob/f2a3eb987e1fc2c85320fc3849c67811f5736b50/examples/utils_squad.py#L511

I do not check there are others unexpectedly behavior or not yet.
I'll update the issue after checking them.

Why do you think the unique_id is not serial?
Each process should convert ALL the dataset.
Only the PyTorch dataset should be split among processes.

By the way it would be cleaner if the other processes wait for the first process to pre-process the dataset before using the cache so the dataset is only converted once and not several time in parrallel (waste of compute). I'll add this option.<|||||>@thomwolf Hi, thanks for your reply

For example, the unique_id should [1000000, 1000001, 1000002, ...]
However, with multi-process I got [1000000, 100001, 1000004, ....]

I did not check what cause the error.
As a result, when predict the answer with multiple gpu, the key error happened.<|||||>![image](https://user-images.githubusercontent.com/13112588/65435142-5fc5c280-ddf6-11e9-86ea-fdeb8eda2ad8.png)

Had the same problema: a KeyError 1000000 after doing a distributed training. Does anyone know how to fix it?<|||||>@ayrtondenner Have the same problem as you when distributed training, after evaluation completes and in writing predictions:

_" File "/media/dn/dssd/nlp/transformers/examples/utils_squad.py", line 511, in write_predictions
    result = unique_id_to_result[feature.unique_id]
KeyError: 1000000000"_

Setup:  transformers 2.0.0; pytorch 1.2.0; python 3.7.4; NVIDIA 1080Ti x 2
_" python -m torch.distributed.launch --nproc_per_node=2 ./run_squad.py \ "_

Data parallel with the otherwise same shell script works fine producing the results below, but of course takes  longer with more limited GPU memory for batch sizes.
Results:
{
  "exact": 81.06906338694418,
  "f1": 88.57343698391432,
  "total": 10570,
  "HasAns_exact": 81.06906338694418,
  "HasAns_f1": 88.57343698391432,
  "HasAns_total": 10570
}

Data parallel shell script:

python ./run_squad.py \
  --model_type bert \
  --model_name_or_path bert-base-uncased \
  --do_train \
  --do_eval \
  --do_lower_case \
  --train_file=${SQUAD_DIR}/train-v1.1.json \
  --predict_file=${SQUAD_DIR}/dev-v1.1.json \
  --per_gpu_eval_batch_size=8 \
  --per_gpu_train_batch_size=8 \
  --gradient_accumulation_steps=1 \
  --learning_rate=3e-5 \
  --num_train_epochs=2 \
  --max_seq_length=384 \
  --doc_stride=128 \
  --adam_epsilon=1e-6 \
  --save_steps=2000 \
  --output_dir=./runs/bert_base_squad1_ft_2<|||||>The problem is that `evaluate()` distributes the evaluation under DDP:
https://github.com/huggingface/transformers/blob/079bfb32fba4f2b39d344ca7af88d79a3ff27c7c/examples/run_squad.py#L216
Meaning each process collects a subset of `all_results` 
but then `write_predictions()` expects `all_results` to have *all the results* 😮 

Specifically, `unique_id_to_result` only maps a subset of ids
https://github.com/huggingface/transformers/blob/079bfb32fba4f2b39d344ca7af88d79a3ff27c7c/examples/utils_squad.py#L489-L491
but the code expects an entry for every feature
https://github.com/huggingface/transformers/blob/079bfb32fba4f2b39d344ca7af88d79a3ff27c7c/examples/utils_squad.py#L510-L511

For DDP evaluate to work `all_results` needs to be collected from all the threads. Otherwise don't allow `args.do_eval` and `args.local_rank != -1` at the same time.
edit: or get rid of the `DistributedSampler` and use `SequentialSampler` in all cases. <|||||>Make sense, do you have a fix in mind @immawatson? Happy to welcome a PR that would fix that.<|||||>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.
<|||||>> The problem is that `evaluate()` distributes the evaluation under DDP:
> https://github.com/huggingface/transformers/blob/079bfb32fba4f2b39d344ca7af88d79a3ff27c7c/examples/run_squad.py#L216
> 
> 
> Meaning each process collects a subset of `all_results`
> but then `write_predictions()` expects `all_results` to have _all the results_ 😮
> Specifically, `unique_id_to_result` only maps a subset of ids
> https://github.com/huggingface/transformers/blob/079bfb32fba4f2b39d344ca7af88d79a3ff27c7c/examples/utils_squad.py#L489-L491
> 
> 
> but the code expects an entry for every feature
> https://github.com/huggingface/transformers/blob/079bfb32fba4f2b39d344ca7af88d79a3ff27c7c/examples/utils_squad.py#L510-L511
> 
> For DDP evaluate to work `all_results` needs to be collected from all the threads. Otherwise don't allow `args.do_eval` and `args.local_rank != -1` at the same time.
> edit: or get rid of the `DistributedSampler` and use `SequentialSampler` in all cases.

That didn't work for me. 