Store steps_per_epoch in Trainer

[hungcs]

This allows training callbacks (i.e. def on_batch_end(self, trainer, progress_tracker, save_path: str, sync_step: bool = True):) to use trainer.steps_per_epoch to render step-level progress bars

[github-actions]

Unit Test Results

       6 files  ±       0         6 suites  ±0   1h 19m 17s ⏱️ - 1m 37s
2 806 tests +2 772  2 793 ✔️ +2 764  12 💤 +7  1 ❌ +1 
2 849 runs  +2 761  2 827 ✔️ +2 755  21 💤 +5  1 ❌ +1 

For more details on these failures, see this check.

Results for commit d1aad75. ± Comparison against base commit 6931fe4.