Chore: First class predictions

[kirahowe]

Changelogs

• This PR extracts all of the normalization and serialization logic that goes into working with predictions into a standalone class for clarity
• Closes Add validators for the structure of predictions supplied to prediction evaluation methods #169
  • Predictions are still accepted in a very generous variety of formats and there no breaking changes introduced here, but the structure of provided predictions is now validated and standardizes, with helpful error messages for common error cases.
• Closes Standardize the expected type of predictions despite differences in the number of tasks and test sets in a benchmark/competition #178
  • Evaluation logic now works with the canonical representation of predictions, for both regular and competition benchmarks.
• Competition documentation is removed from the docs while the feature is gated.

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Checklist:

• Was this PR discussed in an issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR.
• Add tests to cover the fixed bug(s) or the newly introduced feature(s) (if appropriate).
• Update the API documentation if a new function is added, or an existing one is deleted.
• Write concise and explanatory changelogs above.
• If possible, assign one of the following labels to the PR: feature, fix, chore, documentation or test (or ask a maintainer to do it for you).

[cwognum]

This is already giving me peace of mind! 🧘

Seeing it all clearly in one spot did raise some questions. Let me know if any of it doesn't make sense. Happy to elaborate!

[Andrewq11]

Great work @cwognum and @kiramclean 🚀

I think this PR is very clean and adds much needed structure to prediction objects, thank you. I've added some comments but I don't think any are blocking.

[jstlaurent]

This looks very fine indeed. I had some suggestions, nothing blocking.

[jstlaurent]

Looks good! Suggested a fox for the build error.