Vocab Size does not change when adding new tokens
Env:
- `transformers` version: 4.8.2
- Platform: Linux-5.4.104+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.7.10
- PyTorch version (GPU?): 1.9.0+cu102

When adding new tokens to an existing tokenizer, the tokenizer's vocab size variable doesn't change. I believe it should be updated every time the tokens change.

Here is a google colab to reproduce: https://colab.research.google.com/drive/1mC_eSmHOgA_F5fPX7AsUt86jAbC7iSSw?usp=sharing

Specifics:
```python
from transformers import AutoTokenizer
tokenizer = AutoTokenizer.from_pretrained("gpt2")
current_size = tokenizer.vocab_size
tokenizer.add_tokens(["new_token"])
tokenizer.vocab_size, current_size, len(tokenizer.vocab)
```

Outputs: (50257, 50257, 50258)

The same happens when I do the following as well
`tokenizer = AutoTokenizer.from_pretrained("gpt2", additional_special_tokens=["new_token"])`
I think you should use `print(len(tokenizer))` instead of `print(tokenizer.vocab_size)` (as the `vocab_size` is a fixed attribute, referring to the base vocabulary without any additional tokens). Refer to [this](https://github.com/huggingface/transformers/issues/1413#issuecomment-538083512) and [this](https://github.com/huggingface/transformers/blob/2dd9440d0835782e41ae415a68e71fd15051c428/src/transformers/tokenization_utils.py#L161).<|||||>ah okay, didn't realize this was expected behavior. Thanks!