XLMRobertaTokenizerFast producing wrong tokenized output
## Environment info

- transformers` version: 4.2.1
- Platform: Linux-4.19.112+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.7.0+cu101 (False)
- Tensorflow version (GPU?): 2.4.0 (False)
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: no

### Who can help
@mfuntowicz
@stefan-it

## Information

Model I am using is XLM-RoBERTa.
The problem arises when using XLMRobertaTokenizerFast tokenizer. 

The tasks I am working on is token-classification. In order to align the labels with the sub-word units I have used the code snippet provided here: https://huggingface.co/transformers/custom_datasets.html [ Fine-tuning with custom datasets/Token Classification with W-NUT Emerging Entities ]. 

When trying to align the labels with the encodings, it throws: "ValueError: NumPy boolean array indexing assignment cannot assign X input values to the Y output values where the mask is true."

This behavior is due to tokenizing punctuation. Moreover comma ( ' , ' ) gets tokenized into '__' and ',' ( having offset values (0,1) ) Similar behavior happens with dot. However, some other punctuation marks are producing only one token (i.g.  ' : ' -> '__:'). 
In addition, the offset_mapping value for ':' is different in different sentences resulting either in (0,0) or (0,3) tuple. The problem is that padding tokens have offset tuple with values (0,0) which are excluded from alignment, but in this case I have to preserve the punctuation since it is POS tagging problem. 
## To reproduce
```
print("Token: {}  Offset_mapping: {}".format(train_encodings[338].tokens[67], train_encodings[338].offsets[67]))
# Token: ▁...  Offset_mapping: (0, 0)
print("Token: {}  Offset_mapping: {}".format(train_encodings[20].tokens[2], train_encodings[20].offsets[2]))
# Token: ▁...  Offset_mapping: (0, 3)

```
Moreover, although I fixed this issue by writing my own masks, I found new issue: the blank space which denotes start of the word is tokenized as separate token instead of being together with the starting sub-token.

## To reproduce
```
tokenizer = XLMRobertaTokenizerFast.from_pretrained("xlm-roberta-base")
model= XLMRobertaForTokenClassification.from_pretrained("xlm-roberta-base")
s = "Je često kritizirao vladu ."
print(tokenizer.tokenize(s)) 
# output: ['▁Je', '▁često', '▁krit', 'izira', 'o', '▁', 'vlad', 'u', '▁', '.']
```

## Expected behavior

1. Punctuation marks should be consistently tokenized and having offset values different from padding tokens.
2. The first sub-word token should be with preceding blank space everywhere. 

There are two different subjects being discussed here:
 - The tokenization behavior: how punctuation is tokenized, or how the blank spaces are separated from the next token. This is expected behavior and just describes the way this tokenizer (XLMRoberta) works.
 - The offset mappings, which as described here are wrong in some cases. These need to be fixed, and I am going to describe a bit more the problem and how we are going to solve it below.

### Cause

This bug in offset mapping actually affects **all** the fast tokenizers converted from sentencepiece. During the pre-tokenization step, we first split everything on whitespaces (`WhitespaceSplit` pre-tokenizer), and in a second step, we add the `▁` character in front of each word (`Metaspace` pre-tokenizer). This process is accurate in terms of tokenization, but it makes the offset tracking very difficult:
 - All the whitespaces get removed, so we won't have any token pointing back to them.
 - We add a "new" `▁` in front of each word, so these tokens actually point back to the beginning of each word: the first character.

### How to fix it

The initial idea of using the `WhitespaceSplit` in a first step was simply to deduplicate the whitespaces but since it leads to loss of information we'll replace it with the following process:
 - Normalization step that replaces groups of whitespaces with a single one, effectively mapping the single whitespace to the group in the original input.
 - Pretokenization step: we just keep the `Metaspace` pre-tokenizer.

In order to fix this we need to:
1. Update all the `tokenizer.json` files on the hub, and it will be compatible with any version of `transformers` since we introduced these fast tokenizers (3.5.0+).
2. Update all the conversion steps in `transformers` to avoid creating more buggy tokenizers.<|||||>### List of updated tokenizers:

- https://huggingface.co/google/pegasus-xsum
- https://huggingface.co/google/reformer-crime-and-punishment

### These can't be fixed this way:
The following will need a new version of `transformers` with a bugfix in `tokenizers`. We'll need to find a way to rely on the new `tokenizer.json` version only on versions of `transformers` that include this bugfix, as it would break all the previous ones.

- https://huggingface.co/albert-base-v1
- https://huggingface.co/albert-base-v2
- https://huggingface.co/albert-large-v1
- https://huggingface.co/albert-large-v2
- https://huggingface.co/albert-xlarge-v1
- https://huggingface.co/albert-xlarge-v2
- https://huggingface.co/albert-xxlarge-v1
- https://huggingface.co/albert-xxlarge-v2
- https://huggingface.co/camembert-base
- https://huggingface.co/facebook/mbart-large-en-ro
- https://huggingface.co/moussaKam/barthez
- https://huggingface.co/moussaKam/barthez-orangesum-title
- https://huggingface.co/moussaKam/mbarthez
- https://huggingface.co/t5-11b
- https://huggingface.co/t5-3b
- https://huggingface.co/t5-base
- https://huggingface.co/t5-large
- https://huggingface.co/t5-small
- https://huggingface.co/xlm-roberta-base
- https://huggingface.co/xlm-roberta-large
- https://huggingface.co/xlm-roberta-large-finetuned-conll02-dutch
- https://huggingface.co/xlm-roberta-large-finetuned-conll02-spanish
- https://huggingface.co/xlm-roberta-large-finetuned-conll03-english
- https://huggingface.co/xlm-roberta-large-finetuned-conll03-german
- https://huggingface.co/xlnet-base-cased
- https://huggingface.co/xlnet-large-cased<|||||>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/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Unstale<|||||>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/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Any update on this one?<|||||>Bump<|||||>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/master/CONTRIBUTING.md) are likely to be ignored.