XLM-Roberta offset mapping is off by one in case of whitespace-subwords
### System Info

```shell
- `transformers` version: 4.19.2
- Platform: Linux-5.4.0-94-generic-x86_64-with-glibc2.29
- Python version: 3.8.10
- Huggingface_hub version: 0.6.0
- PyTorch version (GPU?): 1.7.1+cu110 (True)
- Tensorflow version (GPU?): 2.9.1 (True)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: <fill in>
```


### Who can help?

@LysandreJik @SaulLu

### Information

- [ ] The official example scripts
- [X] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [X] My own task or dataset (give details below)

### Reproduction

>>> from transformers import AutoTokenizer
>>> tokenizer = AutoTokenizer.from_pretrained('xlm-roberta-large', use_fast=True)
>>> tokenizer.tokenize('Quality of work is sufficient')
['▁Quality', '▁of', '▁work', '▁is', '▁', 'sufficient']
>>> tokenizer.encode_plus('Quality of work is sufficient', return_offsets_mapping=True)
{'input_ids': [0, 124604, 111, 4488, 83, 6, 129980, 2], 'attention_mask': [1, 1, 1, 1, 1, 1, 1, 1], 'offset_mapping': [(0, 0), (0, 7), (8, 10), (11, 15), (16, 18), (19, 20), (19, 29), (0, 0)]}


### Expected behavior

```shell
The third-last offset-tuple (19,20) overlaps with the second-last offset-tuple(19,29). I believe that this should be (18,19), and thus refer to the whitespace.
```

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>I think @robvanderg is right (with my intuitive understanding). Since `return_offsets_mapping` is only for fast tokenizer, let me kindly tag @Narsil here

The results should be like
`... (18, 19), (19, 29) ...`
I think.<|||||>I agree with you @ydshieh, to solve it it might be best to open an issue on the [tokenizers](https://github.com/huggingface/tokenizers) library as this is where the offsets are computed :blush: <|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.