Skip some doctests in quicktour
The quicktour includes code snippets that instantiate a generic `dataset["train"]` and `dataset["test"]` (in the `Trainer` sections) that's only meant to be an example a user can copy/paste and replace with their own dataset. This causes the tests to fail since no dataset is actually being loaded. This PR adds the `# doctest: +SKIP` directive to skip the affected code snippets (the alternative option is to include a real dataset in the examples that can be loaded).
_The documentation is not available anymore as the PR was closed or merged._<|||||>LGTM, but it might be better to use an already processed dataset we could host somewhere so the whole thing can run (particularly since this also becomes a notebook).<|||||>I'll merge this for now so the daily CI is happy and then update it later with a processed dataset!