KeyError in GLUE data tokenization with RoBERTA
# 🐛 Bug

I'm getting a KeyError [here](https://github.com/huggingface/transformers/blob/master/src/transformers/data/processors/glue.py#L94) when using RoBERTa in [examples/run_glue.py](https://github.com/huggingface/transformers/blob/master/examples/run_glue.py) and trying to access `'token_type_ids'` while preprocessing the data, maybe from [this commit](https://github.com/huggingface/transformers/commit/5164ea91a7b4d35cb03867233527fa383a651775) removing `'token_type_ids'` from RoBERTa (and DistilBERT)?

I get the error when fine-tuning RoBERTa on CoLA and RTE. I haven't tried other tasks, but I think you'd get the same error.
I don't get the error when fine-tuning XLNet (presumably, since XLNet does use `'token_type_ids'`), and I don't get the error when I do `pip install transformers` instead of `pip install .` (which I think means the issue is coming from a recent commit).

Here's the full error message:
```bash
03/17/2020 11:53:58 - INFO - transformers.data.processors.glue -   Writing example 0/13997
Traceback (most recent call last):
  File "examples/run_glue.py", line 731, in <module>
    main()
  File "examples/run_glue.py", line 679, in main
    train_dataset = load_and_cache_examples(args, args.task_name, tokenizer, evaluate=False)
  File "examples/run_glue.py", line 419, in load_and_cache_examples
    pad_token_segment_id=4 if args.model_type in ["xlnet"] else 0,
  File "/home/ejp416/cmv/transformers/src/transformers/data/processors/glue.py", line 94, in glue_convert_examples_to_features
    input_ids, token_type_ids = inputs["input_ids"], inputs["token_type_ids"]
KeyError: 'token_type_ids'
```

## Information

Model I am using (Bert, XLNet ...): RoBERTa. I think DistilBERT may run into the same issue as well.

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)

I've made slight modifications to the training loop in the official [examples/run_glue.py](https://github.com/huggingface/transformers/blob/master/examples/run_glue.py), but I did not touch the data pre-processing, which is where the error occurs (before any training).

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

I've run into the error on CoLA and RTE, though I think the error should happen on all GLUE tasks.

## To reproduce

Steps to reproduce the behavior:

1. Install `transformers` using the latest clone (use `pip install .` not `pip install transformers`)
2. Download the RTE data (e.g., into `data/RTE` using the GLUE download scripts in this repo)
3. Run a command to train RoBERTa (base or large). I'm using:
```
python examples/run_glue.py --model_type roberta --model_name_or_path roberta-base --output_dir models/debug --task_name rte --do_train --evaluate_during_training --data_dir data/RTE --max_seq_length 32 --max_grad_norm inf --adam_epsilon 1e-6 --adam_beta_2 0.98 --weight_decay 0.1 --logging_steps 874 --save_steps 874 --num_train_epochs 10 --warmup_steps 874 --per_gpu_train_batch_size 1 --per_gpu_eval_batch_size 2 --learning_rate 1e-5 --seed 12 --gradient_accumulation_steps 16 --overwrite_output_dir
```

## Expected behavior

`load_and_cache_examples` (and specifically, the call to `convert_examples_to_features`) in `examples/run_glue.py` should run without error, to load, preprocess, and tokenize the dataset.

## Environment info

- `transformers` version: 2.5.1
- Platform: Linux-3.10.0-1062.12.1.el7.x86_64-x86_64-with-centos-7.7.1908-Core
- Python version: 3.7.6
- PyTorch version (GPU?): 1.4.0 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: Error happens with both GPU and CPU
- Using distributed or parallel set-up in script?: No

I also have this issue when i run run_multiple_choice.py in RACE data with RoBERTA.<|||||>I get the same error when I try to fine-tune Squad<|||||>Tagging @LysandreJik <|||||>> 
> 
> I also have this issue when i run run_multiple_choice.py in RACE data with RoBERTA.

Same here. Any solution?<|||||>@nielingyun @orena1 @Onur90 maybe try pulling again from the latest version of the repo and see if it works? The error went away after I pulled recently, not sure if that fixed it or something else I did - let me know if that worked<|||||>@ethanjperez by latest version you mean **latest commit** or the **latest release** (v2.6.0)? It is still not working with the **latest commit**.