Max Length rework

[psinger]

This PR adresses several related points:

• It removes the settings max_length_prompt and max_length_answer and only keeps max_length. It will always use that setting to truncate the left side of the whole input.
• It will first truncate all prompts on the left side, and all answers on the right side with max_length.
• It removes the strict requirement for the system prompt to always be on the left side of the input, even if truncated. It will also be truncated if the whole input exceeds max length.
• Removes the strict forced requirement for the last label to always be eos token, if setting is set.
• It is now passing correct position_ids to the forward method of the backbone.

[pascal-pfeiffer]

Thank you @psinger. Great changes!

Some suggestions in the code.
I would also suggest to add a small note in the Readme for recent breaking changes, as depending on the parameters, the new behavior could be different, but should be better in general and resemble the chat_template from transformers.

[pascal-pfeiffer]

Thank you for the quick changes, looks good to me!