Fix logging metrics in DDP mode

[ashleve]

What does this PR do?

Fixes #407

Implements logging metrics through objects instead of through values.

Wraps loss in MeanMetric to unify the way loss and accuracy are logged.

Gets rid of resetting metrics at the end of epoch since it's no longer needed when logging through metric object.

*The main motivation for this PR is no longer needing to set sync_dist=True in DDP mode to synchronize metrics across GPU processes (since synchronization is automatic when logging through metric object).

**Notice logging through metric object and logging through value gives exactly the same results:

self.log("val/loss_v1", loss_value, on_step=True, on_epoch=True)
self.log("val/loss_v2", self.val_loss, on_step=True, on_epoch=True)

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[codecov-commenter]

Codecov Report

Base: 84.59% // Head: 84.81% // Increases project coverage by +0.22% 🎉

Coverage data is based on head (f45f90c) compared to base (2f3fe86).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #426      +/-   ##
==========================================
+ Coverage   84.59%   84.81%   +0.22%     
==========================================
  Files           9        9              
  Lines         344      349       +5     
==========================================
+ Hits          291      296       +5     
  Misses         53       53              

Impacted Files	Coverage Δ
src/models/mnist_module.py	100.00% <100.00%> (ø)

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