Forward `shuffle` to trainer server and measure GPU

[XianzheMa]

1. Add a unit test to how we pack the StartTrainingRequest, thereby catching the unspecified shuffle.
2. Enable accurate GPU operations measurement.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.66667% with 8 lines in your changes missing coverage. Please review.

Project coverage is 82.48%. Comparing base (e6541d3) to head (e1e3eb5).

:exclamation: Current head e1e3eb5 differs from pull request most recent head 5588511

Please upload reports for the commit 5588511 to get more accurate results.

Files	Patch %	Lines
...trainer_server/internal/trainer/pytorch_trainer.py	77.41%	7 Missing :warning:
modyn/common/grpc/grpc_helpers.py	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #526 +/- ## ========================================== + Coverage 82.43% 82.48% +0.04% ========================================== Files 214 215 +1 Lines 10038 10049 +11 ========================================== + Hits 8275 8289 +14 + Misses 1763 1760 -3 ```

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[github-actions[bot]]

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