Leaderboard v2 comp fixes

@ mention of reviewers`

@Didayolo, @Tthomas63`

A brief description of the purpose of the changes contained in this PR.

This PR is for getting the leaderboard to show results from submissions

Issues this PR resolves

When loading the main page for a competition, codabench also loads the leaderboard. There were two issues:

• compute_worker.py: The yaml loader that parses the leaderboard scores needed a "yaml.Loader" argument as well as the file object.
• leaderboards.py: had a column serializer that was only returning leaderboards for competitions with more than one metric. I changed so that it accepts 1 or more metrics.

A checklist for hand testing

• upload comp
• make submission
• add results to leaderboard
• check leaderboard for results

Any relevant files for testing

• Compute Pi: https://github.com/codalab/competition-examples/tree/master/basic-competition-bundles/Compute_pi
• Iris: https://github.com/codalab/competition-examples/tree/master/basic-competition-bundles/Iris

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• Ready to merge