Segmentation fault (core dumped) after importing transformers
# 🐛 Bug

## Information

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

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

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [x] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1.import torch
2.import transformers
3.torch.rand(4,5)

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```python
import torch
import transformers
torch.rand(4,5)
```
## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
I got an error 'segmentation fault (core dumped)' while trying to generate a tensor after importing transformers, but if I removed 'import transformers', the tensor could be generated.
![image](https://user-images.githubusercontent.com/32594371/86000735-83caca00-ba40-11ea-9f25-cf8617f843aa.png)

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 2.11.0
- Platform: Ubuntu 18.04.4 LTS
- Python version: 3.7.6
- PyTorch version (GPU?): 1.4.0+cu100
- Tensorflow version (GPU?): 2.2.0
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

Hi, I believe this error may be due to [the sentencepiece version 0.1.92 which causes a segmentation fault.](https://github.com/huggingface/transformers/issues/4857).<|||||>> Hi, I believe this error may be due to [the sentencepiece version 0.1.92 which causes a segmentation fault.](https://github.com/huggingface/transformers/issues/4857).

Thank you very much, this error disappeared when sentencpiece was downgraded to 0.1.91.
BTW, is there some debugging method to find this root cause when we encounter segmentation fault? <|||||>Not that I know of :man_shrugging: <|||||>It works. GREAT!!!!<|||||>The error still persists after 6 months. Maybe the version 0.1.91 should be hardcoded in package requirements<|||||>It was for the last 3.x versions. For version 4.x onwards, `sentencepiece` is not a requirement anymore.