Reformer tokenizer not working
# 🐛 Bug
`TextDataset` throws out of index error.
## Information
```
model_args = ModelArguments(model_name_or_path = 'Reformer', 
                            config_name = 'ReformerConfig',
                           tokenizer_name = 'ReformerTokenizer')
training_args = TrainingArguments(output_dir = '/tmp', do_train = True,
                                  evaluate_during_training = True,
                                  do_eval = True,
                                  do_predict = False)
data_args = DataTrainingArguments(train_data_file = TRAIN_DATA_DIR,
                                  eval_data_file = EVAL_DATA_DIR,
                                 line_by_line = True)
tokenizer = AutoTokenizer.from_pretrained("google/reformer-crime-and-punishment",
                                         cache_dir=model_args.cache_dir)
train_dataset = (
    get_dataset(data_args, tokenizer=tokenizer, local_rank=training_args.local_rank)
    if training_args.do_train
    else None
)
eval_dataset = (
    get_dataset(data_args, tokenizer=tokenizer, local_rank=training_args.local_rank, evaluate=True)
    if training_args.do_eval
    else None
)
```
It throws this:
```
~/.local/lib/python3.6/site-packages/transformers/tokenization_utils.py in truncate_sequences(self, ids, pair_ids, num_tokens_to_remove, truncation_strategy, stride)
   2044             for _ in range(num_tokens_to_remove):
   2045                 if pair_ids is None or len(ids) > len(pair_ids):
-> 2046                     overflowing_tokens = [ids[-1]] + overflowing_tokens
   2047                     ids = ids[:-1]
   2048                 else:

IndexError: list index out of range
```
When `line_by_line` is kept False, 
```
train_dataset[0]
```
returns 
```
~/.local/lib/python3.6/site-packages/transformers/data/datasets/language_modeling.py in __getitem__(self, i)
     72 
     73     def __getitem__(self, i) -> torch.Tensor:
---> 74         return torch.tensor(self.examples[i], dtype=torch.long)
     75 
     76 

IndexError: list index out of range
```
Model I am using (Bert, XLNet ...): Reformer

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)

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


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## Expected behavior

`TextDataset` should work as per standard Pytorch `Dataset` class.

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 2.9
- Platform: Ubuntu 18.04
- Python version: 3.8
- PyTorch version (GPU?): 1.5
- Tensorflow version (GPU?): NA
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No
