Bug: Tokenization of Special Tokens
## 🐛 Bug

The commit https://github.com/huggingface/transformers/commit/deceb001616995199a6a5dca866ffec95c3ebe74 introduces a bug in the tokenization of special tokens when using `from_pretrained` to initialize the tokenizer.

```
from transformers import AutoTokenizer

bert_tokenizer = AutoTokenizer.from_pretrained('bert-base-uncased')
bert_text = "[CLS] An example for [MASK] change. [SEP]"
bert_tokenizer.tokenize(bert_text)
# Before: ['[CLS]', 'an', 'example', 'for', '[MASK]', 'change', '.', '[SEP]']
# After: ['[', '[UNK]', ']', 'an', 'example', 'for', '[', '[UNK]', ']', 'change', '.', '[', '[UNK]', ']']

roberta_tokenizer = AutoTokenizer.from_pretrained('roberta-base')
roberta_text = "<s> An example for <mask> change. </s>"
roberta_tokenizer.tokenize(roberta_text)
# Before: ['<s>', 'An', 'Ġexample', 'Ġfor', '<mask>', 'change', '.', '</s>']
# After: ['<', 's', '>', 'ĠAn', 'Ġexample', 'Ġfor', 'Ġ<', 'mask', '>', 'Ġchange', '.', 'Ġ</', 's', '>']
````

Fixed by https://github.com/huggingface/transformers/pull/2312.