Add inference section to task guides
Currently, the task guides only show how to finetune a model, but it doesn't directly connect the dots to how you can use that model for inference. For completeness, this PR adds a section to the task guides to show how to use a model for inference after finetuning. This gives users a better overview of the model lifecycle.

In doing so, when we update `task_summary.mdx`, we can focus less on the practical steps of how to use a model for inference (we can add links to the task guides) and instead discuss the more theoretical aspects of these tasks as intended by the Conceptual Guide section.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for the suggestions! I reworked the `sequence_classification` task a bit, and if we like the changes, then I can apply them to the other tasks. Main changes are:

- Encourage users to login with their HF accounts so they can push their finetuned models. Along these lines, I've added `push_to_hub` and the `PushToHubCallback`.
- Added a section to include a `compute_metrics` function so users can evaluate their models during training.

I think adding these two will help the task guides be more complete :)