RobertaTokenizer doesn't have 'batch_encode_plus'
# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...): RoBERTa

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)

Following the tutorial on how to train your own RoBERTa model in this [link](https://huggingface.co/blog/how-to-train)

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

Fill mask

## To reproduce
Steps to reproduce the behavior:

1. I pretrained my own tokenizer and roberta model 
2.  The tokenizer and model are loaded fine and both seems to have the training information 
3. However, when I join them together in the  pipeline step as in:
`tokenizer = RobertaTokenizer.from_pretrained('./eo_data')`
`rmodel = RobertaForMaskedLM.from_pretrained('./output_dir')`
`fill_mask = pipeline(
    "fill-mask",
    model=rmodel,
    tokenizer= tokenizer
)
`

 I get the following error:
`
AttributeError: 'RobertaTokenizer' object has no attribute 'batch_encode_plus'
`
It seems that RobertaTokenizer doesn't have the batch_encode_plus function as BertTokenizer 

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->

## Environment info
<!-- You can run the command `python transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 2.5.1
- Platform: Ubuntu 
- Python version: 3.6.8
- PyTorch version (GPU?):
- Tensorflow version (GPU?):
- Using GPU in script?:
- Using distributed or parallel set-up in script?:

Thank you 

Hi, this is probably because you're using an old version of `transformers`. Version 2.8 doesn't exist, the latest is 2.5.1 ...<|||||>Sorry, I accidently put 2.8. The version I have is: 2.5.1
I am going to edit the post. <|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>Started getting the same issue today. Is there a known solution?