[llama] AutoTokenizer does not add `eos_token` at the end
### System Info

Copy-and-paste the text below in your GitHub issue and FILL OUT the two last points.

- `transformers` version: 4.29.2
- Platform: Linux-3.10.0-1160.42.2.el7.x86_64-x86_64-with-glibc2.35
- Python version: 3.9.16
- Huggingface_hub version: 0.14.1
- Safetensors version: not installed
- PyTorch version (GPU?): 2.0.1+cu118 (True)
- Tensorflow version (GPU?): not installed (NA)
- 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?

@ArthurZucker

### Information

- [ ] The official example scripts
- [X] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [X] My own task or dataset (give details below)

### Reproduction

code:
```python
from transformers import AutoTokenizer, LlamaTokenizer

auto_tokenizer = AutoTokenizer.from_pretrained("huggyllama/llama-7b", add_eos_token=True, use_fast=True)
llama_tokenizer = LlamaTokenizer.from_pretrained("huggyllama/llama-7b", add_eos_token=True, use_fast=True)

print(auto_tokenizer.decode(auto_tokenizer.encode("auto_tokenizer", add_special_tokens = True)))
print(llama_tokenizer.decode(llama_tokenizer.encode("llama_tokenizer", add_special_tokens = True)))
```

results:
```shell
<s> auto_tokenizer
<s> llama_tokenizer</s>
```

### Expected behavior

add eos token like:

```shell
<s> auto_tokenizer</s>
<s> llama_tokenizer</s>
```
Hi,

Note that it doesn't make sense to pass `use_fast` to the slow (Python-based) `LlamaTokenizer`. It only makes sense to pass use_fast to the `AutoTokenizer` class, which can either load the fast (Rust-based) `LlamaTokenizerFast` class or the slow (Python-based) `LlamaTokenizer`.

In the code snippet above, `auto_tokenizer` will be an instance of `LlamaTokenizerFast` and `llama_tokenizer` will be an instance of `LlamaTokenizer`:
```
>>> type(auto_tokenizer)
<class 'transformers.models.llama.tokenization_llama_fast.LlamaTokenizerFast'>
>>> type(llama_tokenizer)
<class 'transformers.models.llama.tokenization_llama.LlamaTokenizer'>
```

Pinging @ArthurZucker regarding the `eos_token` issue<|||||>> Hi,
> 
> Note that it doesn't make sense to pass `use_fast` to the slow (Python-based) `LlamaTokenizer`. It only makes sense to pass use_fast to the `AutoTokenizer` class, which can either load the fast (Rust-based) `LlamaTokenizerFast` class or the slow (Python-based) `LlamaTokenizer`.
> 
> In the code snippet above, `auto_tokenizer` will be an instance of `LlamaTokenizerFast` and `llama_tokenizer` will be an instance of `LlamaTokenizer`:
> 
> ```
> >>> type(auto_tokenizer)
> <class 'transformers.models.llama.tokenization_llama_fast.LlamaTokenizerFast'>
> >>> type(llama_tokenizer)
> <class 'transformers.models.llama.tokenization_llama.LlamaTokenizer'>
> ```
> 
> Pinging @ArthurZucker regarding the `eos_token` issue

Thank you so much for explaining this ~~~<|||||>Hey! 
Thanks for reporting. The quickest fix I can give you is to initialise the fast tokenizer from the slow one, using the correct arguments. 
```python 
fast = LlamaTokenizerFast.from_pretrained("huggyllama/llama-7b", add_eos_token=True, from_slow=True)
```
This will produce the expected outputs: 
```python 
>>>  fast.encode("auto_tokenizer", add_special_tokens = True)
[1, 4469, 29918, 6979, 3950, 2]
```
The reason behind this is that the `post_processor` is responsible of adding the `eos` and `bos` tokens. The processor is initialised when the slow tokenizer is converted to the fast version, and changing the argument on the fly will not result in a change of the processor. 

I'll open a PR to make sure that changing the eos and bos update the processor. Thanks for reporting. 