Add MultipleChoice to TFTrainer [WIP]
Working on getting the TF trainer running on TPUs for multiple choice. I'm quite the newb so input is needed.

- [x]  Add 1 GPU strategy
- [x]  Args for TPU Name
- [x]  Add MPC example for TFTrainer
- [x]  Fix docstrings

Examples:
GPU on Swag
https://colab.research.google.com/drive/1X9d1TY1I7r8d3rCJ9vWiy0UgvJM6BqQ4

TPU on Swag (fails, havent looked into it might be my misstake):
https://colab.research.google.com/drive/1X9d1TY1I7r8d3rCJ9vWiy0UgvJM6BqQ4#scrollTo=2r1Rng8NWzit

Glue on TPU (fails, havent looked into it, might be my mistake)
https://colab.research.google.com/drive/1f2x5wPSaBV0NbGCpMyUPXGUSMEGjSHeX#scrollTo=YDhXmegN3WWj
Hey! 

Thanks a lot for your contribution on the trainer :+1:

Your PR looks great! Just have a small comment, see above. Don't hesitate if you have questions!<|||||>#4252 

```
05/11/2020 19:59:05 - INFO - oauth2client.transport -   Attempting refresh to obtain initial access_token
INFO:tensorflow:Found TPU system:
05/11/2020 19:59:05 - INFO - tensorflow -   Found TPU system:
INFO:tensorflow:*** Num TPU Cores: 8
05/11/2020 19:59:05 - INFO - tensorflow -   *** Num TPU Cores: 8
INFO:tensorflow:*** Num TPU Workers: 1
05/11/2020 19:59:05 - INFO - tensorflow -   *** Num TPU Workers: 1
INFO:tensorflow:*** Num TPU Cores Per Worker: 8
05/11/2020 19:59:05 - INFO - tensorflow -   *** Num TPU Cores Per Worker: 8
```
So its grabbing by name now by specifying --tpu_name<|||||>How do I see what fails with the code quality in __init__.py? I do run make style hehe

Great work with the trainer @jplu I saw your other plans for it. Would be very neat.

Ill mess around with the TFdataset and tpus on another branch, i need to read up on some stuff. Is there anywhere i can see the plans for a framework agnostic approach for the datasets?<|||||>Thanks :)

For the style I always do it manually in this following order:

```
isort --recursive examples templates tests src utils
black --line-length 119 --target-version py35 examples templates tests src utils
flake8 examples templates tests src utils
```
Then git add/commit/push.

What is currently working in your MPC example? Single GPU and CPU?<|||||>> Thanks :)
> 
> For the style I always do it manually in this following order:
> 
> ```
> isort --recursive examples templates tests src utils
> black --line-length 119 --target-version py35 examples templates tests src utils
> flake8 examples templates tests src utils
> ```
> 
> Then git add/commit/push.
> 
> What is currently working in your MPC example? Single GPU and CPU?

Yep only tested on my trusty rtx2080 and in colab on gpu.<|||||>Very Good!

You should also update the examples [README](https://github.com/huggingface/transformers/blob/master/examples/README.md) to add the check to the multiple_choice TFTrainer support :wink: <|||||>Can you also add a TF section in the multiple_choice [README](https://github.com/huggingface/transformers/tree/master/examples/multiple-choice) plz<|||||>> Can you also add a TF section in the multiple_choice [README](https://github.com/huggingface/transformers/tree/master/examples/multiple-choice) plz

I should sleep on this. Thanks alot for the help!<|||||>Looks very good! Let me know once you think it can be merged.<|||||>@ViktorAlm I think it's great if you link to your Colab! Maybe just save it to a repo so that one can take a look at the history (and open issues if it ever breaks:)<|||||>None of the failing flake8 code quality seems to be related to me. If this works I think its done for now.<|||||>Looks good to me!

@julien-c and @LysandreJik should double check and merge if it seems good for them.<|||||>Maybe add a link to the syn dataset you added. It's [that one](https://aclweb.org/aclwiki/TOEFL_Synonym_Questions_(State_of_the_art)) right?<|||||>In the Colab ideally we would expose/expand a little bit more what happens in the example scripts (see what we did in the Colab for GLUE for instance), i.e. the calls to Trainer/TFTrainer.

We can improve/expand this in the future though @ViktorAlm @jplu <|||||>> Maybe add a link to the syn dataset you added. It's [that one](https://aclweb.org/aclwiki/TOEFL_Synonym_Questions_(State_of_the_art)) right?

Its my own dataset which ill release once I'm done