Fix horizon accuracy plots

[dfulu]

Pull Request

Description

Previously the horizon accuracy plots logged to wandb have only been results from a single batch of val data. This corrects the behaviour so that it is the mean over the validation set

Also included some tidying

How Has This Been Tested?

• [x] TODO: test in training run
  • The results from validating a model are here

Checklist:

• [x] My code follows OCF's coding style guidelines
• [x] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I have checked my code and corrected any misspellings