Fix Llama2 implementation: Cannot import name 'Llama2Tokenizer' from 'transformers'

I created a quickstart test file which is mainly doing the same that appears on the documentation:
```python
from llm_vm.client import Client

client = Client(
big_model = 'neo',
big_model_config={'model_uri':'EleutherAI/gpt-neo-1.3B'},
small_model ='neo',
small_model_config={'model_uri':'EleutherAI/gpt-neo-125m'})

response=client.complete(prompt='What is Anarchy?', context='')
print(response)
```

But when I try to run it return it the next error:
```
python quickstart-test.py
Traceback (most recent call last):
File ".../LLM-VM/quickstart-test.py", line 2, in
from llm_vm.client import Client
File ".../LLM-VM/src/llm_vm/client.py", line 3, in
import llm_vm.onsite_llm as llms
File ".../LLM-VM/src/llm_vm/onsite_llm.py", line 6, in
from transformers import (
ImportError: cannot import name 'Llama2Tokenizer' from 'transformers' (.../LLM-VM/.venv/lib/python3.10/site-packages/transformers/__init__.py
```

If I checkout at commit 7a5877b6a everything works fine.

I checked the [huggingface transformers documentation and llama2 model is using the same transformer](https://huggingface.co/docs/transformers/model_doc/llama2) I also checked the [repo](https://github.com/huggingface/transformers/blob/main/src/transformers/models/llama/tokenization_llama.py) and there is no llama2 folder nor Llama2Tokenizer, Llama2ForCausalLM classes.