Fix timer initialization in StepTimeCallback

[yuxin00j]

Updates the StepTimeCallback within the PyTorch Lightning training script to correctly initialize timers at the very beginning of training rather than at the start of each epoch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.68%. Comparing base (25d99d9) to head (1f6a023).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #946   +/-   ##
=======================================
  Coverage   89.68%   89.68%           
=======================================
  Files          16       16           
  Lines        3579     3579           
=======================================
  Hits         3210     3210           
  Misses        369      369           

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[gemini-code-assist]

Code Review

This pull request removes the on_train_epoch_start callback from the PyTorch Lightning training simulation script. The reviewer noted that removing this callback will introduce a bug in multi-epoch training, as the first step of subsequent epochs would incorrectly include inter-epoch overhead, thereby inflating the measured step time. It is recommended to retain both on_train_start and on_train_epoch_start to ensure accurate timing metrics.