data_collator.py does not allow NoneType labels for test set predictions on Glue
# 🐛 Bug

## Information

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

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

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

The tasks I am working on is:
* [x] 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. Open the  example Colab for text-classification from here https://huggingface.co/transformers/examples.html
2. Try to run the prediction function found in run_glue.py to predict on the official Glue test set. 
3. The error is as shown below. 

Earlier, this has worked with the exact same program, ever since an update recently, this error shows up. 

`TypeError                                 Traceback (most recent call last)
<ipython-input-16-9eecdd4d48b1> in <module>()
      2 output_mode = "classification"
      3 
----> 4 predictions = trainer.predict(test_dataset=test_dataset).predictions
      5 if output_mode == "classification":
      6     predictions = np.argmax(predictions, axis=1)

7 frames
/usr/local/lib/python3.6/dist-packages/transformers/data/data_collator.py in default_data_collator(features)
     45     if "label" in first:
     46         dtype = torch.long if type(first["label"]) is int else torch.float
---> 47         batch["labels"] = torch.tensor([f["label"] for f in features], dtype=dtype)
     48     elif "label_ids" in first:
     49         if isinstance(first["label_ids"], torch.Tensor):

TypeError: must be real number, not NoneType`

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## Expected behavior
The error can be seen in the Colab notebook here https://colab.research.google.com/drive/1H_92qdsOOql2hS210qNrfMEEMRcAoHD_?usp=sharing

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

## 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:
- Platform: Colab
- Python version: NA
- PyTorch version (GPU?):
- Tensorflow version (GPU?):
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?:
