[BUG] Tokenizer offset
```python
from transformers import AutoTokenizer
tokenizer = AutoTokenizer.from_pretrained('xlm-roberta-base')
text = "k , u , o , 105 , 957 , 601 . "
out = tokenizer.encode_plus(text=text,
                            truncation=True,
                            max_length=128,
                            return_tensors='tf',
                            return_token_type_ids=True,
                            return_attention_mask=True,
                            return_offsets_mapping=True,
                            return_length=True)

print(tokenizer.tokenize(text))
text_offsets = out['offset_mapping'][0].numpy().tolist()
for i in text_offsets:
    s,e = i
    print(f"{(s,e)}:{text[s:e]}")
```

```
['▁k', '▁', ',', '▁u', '▁', ',', '▁o', '▁', ',', '▁105', '▁', ',', '▁9', '57', '▁', ',', '▁', '601', '▁', '.']
(0, 0):
(0, 1):k
(2, 3):,
(2, 3):,
(4, 5):u
(6, 7):,
(6, 7):,
(8, 9):o
(10, 11):,
(10, 11):,
(12, 15):105
(16, 17):,
(16, 17):,
(18, 19):9
(19, 21):57
(22, 23):,
(22, 23):,
(24, 25):6
(24, 27):601
(28, 29):.
(28, 29):.
(0, 0):
```
the offset  can't  return the Original text
1.the offset of '▁' will be  filled  by next token .   ????
2.  (24, 25):6   ????
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