run_tf_glue.py breaks when changing to a glue dataset different from mrpc
## 🐛 Bug - run_tf_glue.py breaks when changing to a glue dataset different from mrpc

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[run_tf_glue.py](https://github.com/huggingface/transformers/blob/master/examples/run_tf_glue.py) breaks when changing to a glue dataset different from `mrpc`, where the features are not called `'sentence1'` and `'sentence2'`. That happens because of the hard coded accesses in the tensor_dict https://github.com/huggingface/transformers/blob/ca559826c4188be8713e46f191ddf5f379c196e7/transformers/data/processors/glue.py#L83

The tasks I am working on is:
* [x] an official GLUE/SQUaD task: SST-2
* [ ] my own task or dataset: (give details)

## To Reproduce

Steps to reproduce the behavior:

1. Go to https://github.com/huggingface/transformers/blob/master/examples/run_tf_glue.py#L11 
2. Change `mrpc` to `sst-2`
3. 💥BOOM! broken

## Expected behavior
* [ ] Handle all glue datasets from `tensorflow_datasets` correctly

P.S.: A colleague and I are currently working on a fix and will submit a PR for this issue in the next couple of minutes.

Fixed with #1355