LayoutXLM tokenizer issues after last update
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->

- `transformers` version: 4.10.0
- Platform: Darwin-20.4.0-x86_64-i386-64bit
- Python version: 3.7.11
- PyTorch version (GPU?): 1.9.0 (False)
- Tensorflow version (GPU?): 2.4.0 (False)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no


### Who can help
@NielsRogge 


**Describe the bug**
I'm using the LayoutXLM. After the last update on huggingface, the tokenizer stopped working correctly.

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [x] my own modified scripts: (give details below)

The tokenizer for `layoutxlm-base` has some mismatched token ids. They exceed the declared tokenizer vocabulary size, and they also have larger ids than the size of the embedding module in model.


**To Reproduce**
Steps to reproduce the behavior:
```
from transformers import AutoTokenizer
tokenizer = AutoTokenizer.from_pretrained('microsoft/layoutxlm-base')
input_ = tokenizer(["foo"], boxes=[[0,0,12,12]], add_special_tokens=True) 
```
This produces
```
ValueError: Id not recognized
```

Tokenization works with `add_special_tokens=True`, but obviously adding special tokens manually causes the model to crash because of too large ids.

**Expected behavior**
Tokenization works with `add_special_tokens=True`, and the model's embeddings are adapted to new changes if necessary.



Hi,

See https://github.com/NielsRogge/Transformers-Tutorials/issues/50#issuecomment-960502393<|||||>Update: I've restored the `tokenizer_class` attribute of the configuration of LayoutXLM, such that your tokenizer still works as expected.

However, once a new version of Transformers comes out, one can use `LayoutXLMTokenizer`/`LayoutXLMTokenizerFast` and the corresponding `LayoutXLMProcessor`, which allow you to prepare all the data for the model (see PR #14115). <|||||>Thanks. I've just tested the changes and it works. 