DistilBertTokenizer for distilbert-base-multilingual-cased is unable to encode / decode Japanese characters properly adding unnecessary characters in between
I have used a google collab notebook.
`transformers` version used is 4.9.2

Model used :
distilbert-base-multilingual-cased 
@LysandreJik
I am facing an issue with the tokenizer: @LysandreJik 

The problem arises when using the tokenizer in the case of Japanese text. I have attached an example script of what I am going to suggest
 
![image](https://user-images.githubusercontent.com/23450481/131085602-be7ffceb-c408-4243-92b4-8abd99b4ec5f.png)

I wanted to obtain the token ids for string "祝い めでた 動画". When I used the list of token ids to obtain the corresponding string, I obtained another string "祝い めでた 動 画" seems like there  is a bug in either of the functions.

Steps to reproduce the behavior:
```python
from transformers import DistilBertTokenizer
import torch
tokenizer = DistilBertTokenizer.from_pretrained('distilbert-base-multilingual-cased')
print(tokenizer.decode([  101, 5914, 15221, 1965, 12236, 20058, 2621, 115384, 102]))
print(tokenizer.decode([  101,  5914,  1906,  1965, 12236, 20058,  2621,  5618,   102]))
inputs = tokenizer("祝い めでた 動画", return_tensors="pt")
print(inputs)

```

Expected behavior:
I should have obtained the same string when I was decoding the token ids obtained from the input string.



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.