Store val metrics in Tensorboard correctly

[georgeyiasemis]

Validation metrics first reduced, then add to Tensorboard if main process

[codecov[bot]]

Codecov Report

Merging #222 (1bd4eac) into main (1a8ddf5) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #222   +/-   ##
=======================================
  Coverage   82.02%   82.02%           
=======================================
  Files          74       74           
  Lines        5585     5587    +2     
=======================================
+ Hits         4581     4583    +2     
  Misses       1004     1004           

Impacted Files	Coverage Δ
direct/engine.py	75.36% <100.00%> (+0.14%)	:arrow_up:

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