update the arguments `add_prefix_space` and `trim_offsets` in `backend_tokenizer.post_processor` of `RobertaTokenizerFast`
Author: SaulLu
Pull request: yes

# What does this PR do?

Roberta's tokenizer fast has `add_prefix_space` and `trim_offsets` as arguments. It seems to me that these last 2 arguments should be updated accordingly in the post processor `RobertaProcessing` of its `backend_tokenizer`. 

This PR proposes to automate this update with a strategy similar to the update of the `pre_tokenizer` done inside the `__init__` of `GPT2TokenizerFast` (from which `RobertaTokenizerFast` inherits).

It is the issue #14305 that allowed me to notice that this update was not done when changing `add_prefix_space` or `trim_offsets`.

I would ideally like to wait until I have an opinion on [this issue](https://github.com/huggingface/tokenizers/issues/843) on the Tokenizers library before merging this PR.

Edit: So I had the confirmation that there was a bug on the Tokenizer side, which was solved in [this PR](https://github.com/huggingface/tokenizers/pull/844) which confirms my understanding of the `RobertaTokenizerFast` component.
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Comments:
Is this leveraging PR https://github.com/huggingface/tokenizers/pull/844 that was just merged in `tokenizers` ? I believe it isn't part of a version yet, right?

Or is this meant as a patch on top of the current tokenizers version?

@LysandreJik, you do raise an important point.

This PR only proposes to reflect the choices of the user on the arguments `add_prefix_space` and `trim_offsets` on the `backend_tokenizer`. Before this modification the `add_prefix_space` and `trim_offsets` arguments were not changed in the post_processors while they are in the pre_tokenizer (in the `__init__` of the GPT2 tokenizer), which causes problems.

Nevertheless, the investigation of this problem allowed us to identify a problem on the side of the Tokenizers library that has been solved in the PR you mention. As long as there is no new version of Tokenizers, this problem will also affect the transformers library.

To sum up,  this PR just aligns the arguments chosen between the pre_tokenizer and the post_processor components of the tokenizer.

To complete this PR, I created [a new branch with a new test that shows the behavior that will remain incorrect](https://github.com/SaulLu/transformers/pull/1) until we can use a new version of `Tokenizers` that includes [this commit](https://github.com/huggingface/tokenizers/pull/844). 

cc @LysandreJik and @sgugger for visibility