Enable option for subword regularization in `XLMRobertaTokenizer`
# What does this PR do?
I would like to use [subword regularization](https://github.com/google/sentencepiece#subword-regularization-and-bpe-dropout) from [google/sentencepiece](https://github.com/google/sentencepiece). The reason is that it might be used to improve downstream task performance.

Since `XLMRobertaTokenizer` already uses `SentencePieceProcessor` from `google/sentencepiece` there are only some minor modifications needed.

3 additional parameters are added to the constructor of `XLMRobertaTokenizer`. This are:
```python
        enable_sampling=False,
        nbest_size=-1,
        alpha=0.1,
```
The default values are selected so that this is no breaking change.

In the `_tokenize(self, text)` function there was a call to `self.sp_model.EncodeAsPieces(text)`. This call does ignore the parameters for subword regularization. That is why it had to be replaced by a call to `self.sp_model.encode(text, out_type=str)`.

Since `XLMRobertaTokenizerFast` is an independent implentation which does not use `google/sentencepiece` it is not in the scope of the PR to add subword regularization to the fast tokenizer.

## To-do
- [x] check if tests pass
- [x] check if tests can / should be added
- [x] add a link to a page where we can see all kwargs

## Who can review?
@LysandreJik @stefan-it 
It would be awesome if you ( @LysandreJik and @stefan-it ) could give some feedback on this - although some tests still fail.
Is it a good idea? Would you merge it if everything is cleaned up?<|||||>I added a test, everything is green and IMO ready for review. @LysandreJik @stefan-it<|||||>@LysandreJik and @n1t0 I think that would be a good idea but IMO it should not be done in the scope of this PR.

Because the slow tokenizer just delegates the work to [google/sentencepiece](https://github.com/google/sentencepiece) this PR is very easy but adding that to the Rust Tokenizer would be way more work afaik.<|||||>Hey @LysandreJik and @n1t0 
I think this PR is somehow stuck...
AFAIK my change is ok for you. 
What about merging it and moving the part for the fast tokenizer to a seperate issue?<|||||>@LysandreJik 
- Requested changes are made and marked as resolved. 
- Inline questions are answered and marked as resolved.
- CI is green.

IMO ready for merge.<|||||>@sgugger all green again :-)<|||||>> Perfect! If you feel up to the task, I think all (slow) sentencepiece-based tokenizers could benefit from this addition.

see #11417