Can't use add_prefix_space in RobertaTokenizerFast
Author: yujianll
Pull request: no

## 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: 4.11.2
- Platform: Linux-5.11.0-34-generic-x86_64-with-glibc2.31
- Python version: 3.9.1
- PyTorch version (GPU?): 1.7.1 (True)
- Tensorflow version (GPU?): not installed (NA)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no

### Who can help
<!-- Your issue will be replied to more quickly if you can figure out the right person to tag with @
 If you know how to use git blame, that is the easiest way, otherwise, here is a rough guide of **who to tag**.
 Please tag fewer than 3 people.

Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @Rocketknight1

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

Model hub:

- for issues with a model report at https://discuss.huggingface.co/ and tag the model's creator.

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

 -->
- tokenizers: @LysandreJik
- Documentation: @sgugger

## Information

Model I am using: RobertaTokenizerFast

The problem arises when using RobertaTokenizerFast with add_prefix_space:
```
tokenizer = RobertaTokenizerFast.from_pretrained('roberta-base')
inputs = tokenizer("This is a sentence.", add_prefix_space=True)
```
it gives me error: `TypeError: _batch_encode_plus() got an unexpected keyword argument 'add_prefix_space'`

However, the doc says 

> You can get around that behavior by passing add_prefix_space=True when instantiating this tokenizer or when you call it on some text

I wonder how I can specify this when I call the tokenizer on some text.

<!-- If you have code snippets, error messages, stack traces please provide them here as well.
     Important! Use code tags to correctly format your code. See https://help.github.com/en/github/writing-on-github/creating-and-highlighting-code-blocks#syntax-highlighting
     Do not use screenshots, as they are hard to read and (more importantly) don't allow others to copy-and-paste your code.-->

## Expected behavior
Be able to pass in `add_prefix_space=True` after initializing the tokenizer, just like RobertaTokenizer.
<!-- A clear and concise description of what you would expect to happen. -->


Comments:
Hi there! Instantiating is different from calling, it means creating the object. So you create the tokenizer:
```
tokenizer = RobertaTokenizerFast.from_pretrained('roberta-base', add_prefix_space=True)
```

Thanks! I see the difference, I'm trying ask if we can instantiate tokenizer without `add_prefix_space=True` and call it with `add_prefix_space=True`. Because the  doc seems to say we can do that. And also we can do that for `RobertaTokenizer`.

No, the latter is not possible. I see what you were saying at first, the doc should indeed be adapted to remove the part "when you call it"