Add end_with_user and include_system_prompt flags to Magpie tasks and handle Nones.

[gabrielmbmb]

Description

This PR updates MagpieBase class to:

• add a new attribute end_with_user that allows ending the conversation generated by Magpie tasks with a user message.
• add a new attribute include_system_prompt which can be used to include or not the used system prompt in the generated conversation
• updates the code to handle None from the LLM.generate method when generating a multi-turn conversation

[github-actions]

Documentation for this PR has been built. You can view it at: https://distilabel.argilla.io/pr-784/

[codspeed-hq]

CodSpeed Performance Report

Merging #784 will not alter performance

_{Comparing magpie-end-with-flag (01a4bad) with develop (b22a494)}

Summary

✅ 1 untouched benchmarks