glue_convert_examples_to_features not working if no task is provided
## 🐛 Bug

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The problem arise when using:
* [X] my own modified scripts: (give details)

The tasks I am working on is:
* [X] my own task or dataset: (give details)

In these lines:
https://github.com/huggingface/transformers/blob/04c69db399b2ab9e3af872ce46730fbd9f17aec3/transformers/data/processors/glue.py#L66-L69

If no task is provided (even if it's a optional parameter), no processor is selected, label_list is still none.

At least an error should be thrown.


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