Can't use add_prefix_space in RobertaTokenizerFast
## Environment info
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- `transformers` version: 4.11.2
- Platform: Linux-5.11.0-34-generic-x86_64-with-glibc2.31
- Python version: 3.9.1
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- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no

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## 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.

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## 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. -->

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"