wrong 'label2id' and 'id2label' in config when loading from pretrained
# 🐛 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. 
```python
from transformers import BertConfig
config = BertConfig.from_pretrained('bert-base-cased', num_labels=3)
print(config.id2label)
```
2. Prints: {0: 'LABEL_0', 1: 'LABEL_1'}


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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->

prints {0: 'LABEL_0', 1: 'LABEL_1', 2: 'LABEL_2'}

## Environment info
<!-- You can run the command `python transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 2.5.1
- Platform: Ubuntu 16.04
- Python version: 3.7.6
- PyTorch version (GPU?): 1.4.0
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no
