Bug in ElectraForTokenClassification
# 🐛 Bug

## Information

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

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

The problem arises when using:
* [√] the official example scripts: (give details below)
  huggingface.co/transformers/model_doc/electra.html#electrafortokenclassification
* [×] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ×] an official GLUE/SQUaD task: (give the name)
* [ √] my own task or dataset: (give details below)
  Ran the example given in the documentation


        from transformers import ElectraTokenizer, ElectraForTokenClassification  
        import torch  

        tokenizer = ElectraTokenizer.from_pretrained('google/electra-small-discriminator')  
        model = ElectraForTokenClassification.from_pretrained('google/electra-small-discriminator')  

        input_ids = torch.tensor(tokenizer.encode("Hello, my dog is cute",   
        add_special_tokens=True)).unsqueeze(0)  # Batch size 1  
        labels = torch.tensor([1] * input_ids.size(1)).unsqueeze(0)  # Batch size 1  
        outputs = model(input_ids, labels=labels)  

        loss, scores = outputs[:2]
## To reproduce

Steps to reproduce the behavior:

1. Start Google Colab
2. Install Transformer and  requriements
3. run the code

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## Expected behavior
AttributeError: 'ElectraForTokenClassification' object has no attribute 'num_labels'
<!-- A clear and concise description of what you would expect to happen. -->

## 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: 2.8.0
- Platform: Windows 10
- Python version: 3.7.6
- PyTorch version (GPU?): Colab
- Tensorflow version (GPU?): Colab
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

Oh no, it should be `self.config.num_labels` instead of `self.num_labels` [here](https://github.com/huggingface/transformers/blob/master/src/transformers/modeling_electra.py#L665)!<|||||>My bad, it should be fixed now!