Tokenizers setter of ids of special tokens don't work
## Environment info
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- `transformers` version:
- Platform:
- Python version:
- PyTorch version (GPU?):
- Tensorflow version (GPU?):
- Using GPU in script?:
- Using distributed or parallel set-up in script?:

### Who can help
- Tokenizers: @SaulLu

## Information

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

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. Create an instance of a pretrained tokenizer
2. Try to set the pad_token_id

For instance:

```
tokenizer = AutoTokenizer.from_pretrained('gpt2')
tokenizer.pad_token_id = tokenizer.eos_token_id
```

Output:
```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_33/1516894257.py in <module>
      1 tokenizer = AutoTokenizer.from_pretrained('gpt2')
----> 2 tokenizer.pad_token_id = tokenizer.eos_token_id

/opt/conda/lib/python3.7/site-packages/transformers/tokenization_utils_base.py in pad_token_id(self, value)
   1173     @pad_token_id.setter
   1174     def pad_token_id(self, value):
-> 1175         self._pad_token = self.convert_tokens_to_ids(value)
   1176 
   1177     @cls_token_id.setter

/opt/conda/lib/python3.7/site-packages/transformers/tokenization_utils_fast.py in convert_tokens_to_ids(self, tokens)
    248 
    249         ids = []
--> 250         for token in tokens:
    251             ids.append(self._convert_token_to_id_with_added_voc(token))
    252         return ids

TypeError: 'int' object is not iterable
```

## Expected behavior

Set the `pad_token` appropriately.

I've fixed this in a branch and I'm submitting a PR.

Thank you very much for sharing this problem (and a solution)! :hugs: 
You are right, this behaviour is not desirable