Feature: Client support for competitions

[Andrewq11]

Changelogs

• Client methods for downloading a competition from the Hub and submitting predictions
• New CompetitionSpecification class that inherits from the DatasetV2 class
• New CompetitionPredictions class that inherits from the BenchmarkPredictions class for standardized prediction uploads
• Removal of old competition code (e.g. differing URLs based on the ArtifactSubtype)
• Removal of the CompetitionResults class since these results never reach the client currently
• New method for loading a competition that accepts a local Zarr path for cached dataset downloads

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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).

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Important Notes

This PR is ready in terms of features, but is currently missing:

• Documentation/tutorials for these changes
• Updated tests

I will leave this PR as a draft to gather comments while I work on these next.

[cwognum]

Thanks @Andrewq11 !

I left some comments. Nothing major!

[Andrewq11]

@cwognum going to mark this ready for review despite missing those couple docs and tutorial. Everything else should be ready 💪

[jstlaurent]

Overall, nice work. I did have some questions/suggestions. Since Andrew is out, I can take a stab at implementing them.

[cwognum]

@jstlaurent If you have the time, it would be great if you could take a stab at this. I was planning to take over from Andrew, but there are some other things on my plate I would really like to wrap before EOY so we can hit the ground running in 2025.

[jstlaurent]

Minor nitpick. Looks good. 🚢 it!