fix loss for batches examples

[peterdudfield]

Pull Request

Description

fix losses, so that don't add up over batches

Fixes #

How Has This Been Tested?

unittests (added new ones)

• [ ] No
• [ ] Yes

Checklist:

• [ ] My code follows OCF's coding style guidelines
• [ ] 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
• [ ] I have checked my code and corrected any misspellings

[codecov-commenter]

Codecov Report

Merging #76 (38b3d02) into main (ed4fe3b) will increase coverage by 0.03%. The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
+ Coverage   78.49%   78.53%   +0.03%     
==========================================
  Files          21       21              
  Lines        1074     1076       +2     
==========================================
+ Hits          843      845       +2     
  Misses        231      231              

Impacted Files	Coverage Δ
nowcasting_utils/visualization/visualization.py	86.20% <66.66%> (ø)
nowcasting_utils/models/loss.py	83.95% <100.00%> (+0.40%)	:arrow_up:
nowcasting_utils/version.py	100.00% <100.00%> (ø)

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