Tracking full integration for fill-in-middle (FIM)

Given that most code LLMs are trained using the FIM objective [1], I think it makes a lot of sense to work on: * A training example just like how we have it here: https://github.com/huggingface/transformers/tree/main/examples/pytorch. This can also be turned into a task guide later (example: https://huggingface.co/docs/transformers/tasks/language_modeling). * A dedicated pipeline so that users can load FIM-trained models easily with `pipeline("fill-in-middle")`. * A task page to list all the relevant resources. Cc @ArthurZucker since we discussed it internally via Slack. **References** [1] Efficient Training of Language Models to Fill in the Middle, https://arxiv.org/abs/2207.14255.