GLUE test set predictions
# 🚀 Feature request

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## Motivation
The `run_glue` script is super helpful.  But it currently doesn't implement producing predictions on the test datasets for the GLUE tasks.  I think this would be extremely helpful for a lot of people.  I'm sure there are plenty of people who have implemented this functionality themselves, but I haven't found any.  Since `transformers` already provides train and dev for GLUE, it would be cool to complete the feature set with providing test set predictions.

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## Your contribution

I'm personally working on a branch that extends the `glue_processors` to support the test sets (which are already downloaded by the recommended `download_glue.py` script.  I also update the `run_glue.py` script to produce the `*.tsv` files required by the GLUE online submission interface.

I think I'm a couple days out from testing/completing my implementation.  I'm also sure plenty of implementations exist of this.  If there are no other plans to support this in the works, I'm happy to submit a PR.

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hi @shoarora 

can you share the script to to report performance on test set?<|||||>@Mahmedturk you can check out the branch in PR #3405 

It's diverge pretty heavily from master and I haven't updated it yet, but you should still be able to run `run_glue.py` off that branch with the `--do_test` flag that I added and it should produce the `.tsv` files required for submission.  <|||||>@shoarora I pulled the repo to update run_glue.py as I wanted to use this new feature. However, I now get an error when I run run_glue.py! Please see below the output of the error message. It looks like in previous versions, there weren't any keyword arguments named "mode" in GlueDataset() --possible? 

`Traceback (most recent call last):
  File "./transformers/examples/text-classification/run_glue.py", line 228, in <module>
    main()
  File "./transformers/examples/text-classification/run_glue.py", line 139, in main
    test_dataset = GlueDataset(data_args, tokenizer=tokenizer, mode="test") if training_args.do_predict else None
TypeError: __init__() got an unexpected keyword argument 'mode'`<|||||>@AMChierici I didn't author #4463, which is what has made it to master to enable this feature.  I haven't played with it yet so sorry I can't be of more help<|||||>@AMChierici make sure you run from master, there's indeed a `mode` kwarg now.

@shoarora Thanks for this first PR and I did check yours while merging the other (to make sure that the indices in csv parsing, etc. were correct)<|||||>Thanks, @julien-c . Yes, solved.. In fact, I was not running from master.
<|||||>downloaded master right now.
  File "examples/text-classification/run_glue.py", line 143, in main
    if training_args.do_eval
TypeError: __init__() got an unexpected keyword argument 'mode'