Added a format based on Huggingface format

[yoavkatz]

Formats the complete input for the model using the Hugginface chat template of a given model.

HFSystemFormat expects the input instance to contain:
1. A field named "system_prompt" whose value is a string (potentially empty) that delivers a task independent opening text.
2. A field named "source" whose value is a string verbalizing the original values in the instance (as read
from the source dataset), in the context of the underlying task.
3. A field named "instruction" that contains a (non-None) string.
4. A field named with the value in arg 'demos_field', containing a list of dicts, each dict with fields "source"
and "target", representing a single demo.
5. A field named "target_prefx" that contains a string to prefix the target in both each demo, and to end the whole generated prompt

SystemFormat formats the above fields into a single string to be inputted to the model. This string overwrites
field "source" of the instance.

Example:
    HFSystemFormat(model_name="HuggingFaceH4/zephyr-7b-beta")

    Uses the template defined the in tokenizer_config.json of the model:

    "chat_template": "{% for message in messages %}\n{% if message['role'] == 'user' %}\n{{ '<|user|>\n' + message['content'] + eos_token }}\n{% elif message['role'] == 'system' %}\n{{ '<|system|>\n' + message['content'] + eos_token }}\n{% elif message['role'] == 'assistant' %}\n{{ '<|assistant|>\n'  + message['content'] + eos_token }}\n{% endif %}\n{% if loop.last and add_generation_prompt %}\n{{ '<|assistant|>' }}\n{% endif %}\n{% endfor %}",

    See more details in https://huggingface.co/docs/transformers/main/en/chat_templating

[yoavkatz]

This PR achieve its goal. For future we should consider: (1) Seperate the structured representation and make it available at a dedicated field for people want to use it externally (e.g for using open ai api) (2) change existing formats to use this mechanism

There are downsides for it - it requires access to the HF model page (e.g. sometime requires huggingface token login ). I think we should consider a general jinja format - so people can just copy the jinja string and use it for formatting.