Update MF submission format for multi-world challenge

[wqi]

PR Summary

This PR updates the submission format for the motion forecasting challenge in preparation for the shift to multi-world evaluation in this year's CVPR challenge.

All new submissions to both the single-agent and multi-world phases of the motion forecasting challenge will be required to use this new format - which should only require slight modifications to the final step of existing methods.

This PR also introduces several new metrics to evaluate multi-world prediction, which build upon their existing single-agent counterparts.

Testing

Updated existing unit tests for metrics and the submission format. Unit tests for new multi-world metrics will be added in a follow-up PR.

In order to ensure this PR works as intended, it is:

• unit tested.
• other or not applicable (additional detail/rationale required)

Compliance with Standards

As the author, I certify that this PR conforms to the following standards:

• Code changes conform to PEP8 and docstrings conform to the Google Python style guide.
• A well-written summary explains what was done and why it was done.
• The PR is adequately tested and the testing details and links to external results are included.