Fix bugs when LayoutLMv3Tokenizer use model microsoft/layoutlmv3-base-chinese
# What does this PR do?
`microsoft/layoutlmv3-base-chinese` **use `sentencepiece` tokenizer** file "sentencepiece.bpe.model" from XLMRoberta instead of "tokenizer.json", when `LayoutLMv3Tokenizer` load this model with `LayoutLMv3Tokenizer.from_pretrained("microsoft/layoutlmv3-base-chinese")`, it will raise exception as follow:

```
[/usr/local/lib/python3.7/dist-packages/transformers/models/layoutlmv3/tokenization_layoutlmv3.py](https://localhost:8080/#) in __init__(self, vocab_file, merges_file, errors, bos_token, eos_token, sep_token, cls_token, unk_token, pad_token, mask_token, add_prefix_space, cls_token_box, sep_token_box, pad_token_box, pad_token_label, only_label_first_subword, **kwargs)
    322         )
    323 
--> 324         with open(vocab_file, encoding="utf-8") as vocab_handle:
    325             self.encoder = json.load(vocab_handle)
    326         self.decoder = {v: k for k, v in self.encoder.items()}

TypeError: expected str, bytes or os.PathLike object, not NoneType
```

 I have tried to fix this bugs by merge code from `XLMRobertaTokenizer`.

And also, after `LayoutLMv3Tokenizer` changed,  `LayoutLMv3Converter` should also be updated to convert to fast tokenizer.

I have write a new test file to test chinese tokenizer.

!!!NOTICE
I have found some differences when processing `chinese` because of `sentencepiece`. 

For example, when we process english document, we have a bound box text `hello word` consist of words `["hello", "word"]`, we tokenize by the words `["hello", "word"]`, get token ids`[[42891], [14742]]`, this is absolutely ok.

But if we have a chinese bound box text `汇丰` consist of words `["汇", "丰"]`, we tokenize by words `["汇", "丰"]`, will get token ids `[[6, 47360], [6, 49222]]`, this is a little strange, **both token ids has `6`, token id `6` is a "▁" (U+2581) , but we do not input `▁`**. It is because **sententcepiece will add space at the begining**(refer [this](https://github.com/google/sentencepiece/issues/15))

So the import thing is that, **when use chinese tokenizer, do not use bound box words as input, use bound box text as input**, here is `["汇丰"]`


```python
from transformers import AutoTokenizer
eng_tok = AutoTokenizer.from_pretrained("roberta-base") # microsot/layoutlmv3-base refer roberta-base
eng_tok(["hello", "word"], add_special_tokens=False)["input_ids"] # [[42891], [14742]]

xlm_tok = AutoTokenizer.from_pretrained("xlm-roberta-base") # microsoft/layoutlmv3-base-chinse refer xlm-roberta-base
xlm_tok(["汇", "丰"], add_special_tokens=False)["input_ids"] # [[6, 47360], [6, 49222]]

xlm_tok = AutoTokenizer.from_pretrained("xlm-roberta-base")
xlm_tok(["汇丰"], add_special_tokens=False)["input_ids"] # [[6, 47360, 49222]]
```

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_The documentation is not available anymore as the PR was closed or merged._<|||||>Hi,

Could you try using [LayoutXLMTokenizer](https://huggingface.co/docs/transformers/model_doc/layoutxlm#transformers.LayoutXLMTokenizer)/[LayoutXLMTokenizerFast](https://huggingface.co/docs/transformers/model_doc/layoutxlm#transformers.LayoutXLMTokenizerFast) instead?

Normally, these should be compatible with microsoft/layoutlmv3-base-chinese.<|||||>> Hi,
> 
> Could you try using [LayoutXLMTokenizer](https://huggingface.co/docs/transformers/model_doc/layoutxlm#transformers.LayoutXLMTokenizer)/[LayoutXLMTokenizerFast](https://huggingface.co/docs/transformers/model_doc/layoutxlm#transformers.LayoutXLMTokenizerFast) instead?
> 
> Normally, these should be compatible with microsoft/layoutlmv3-base-chinese.

@NielsRogge sorry, I didn't find that `LayoutXLMTokenizer` is used for this model, maybe this will make code easier，but maybe   more document is better. I will close this pr