Record training loss

[XianzheMa]

Per the PR title, this PR solves issue https://github.com/eth-easl/modyn/issues/488.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.80%. Comparing base (42354cf) to head (328be5a). Report is 2 commits behind head on main.

Files	Patch %	Lines
...trainer_server/internal/trainer/pytorch_trainer.py	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #539 +/- ## ========================================== - Coverage 82.81% 82.80% -0.01% ========================================== Files 217 217 Lines 10152 10157 +5 ========================================== + Hits 8407 8411 +4 - Misses 1745 1746 +1 ```

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[github-actions[bot]]

^{( % to main) ( % to main)}