refactor : replace iteration with consumed_samples

- Replace context.iteration with consumed_samples (global sample count)
- Add optimizer_step property derived from consumed_samples
- Checkpoint meta.json stores consumed_samples, drops iteration
- CLI --start_batch renamed to --start_samples (per-rank samples)
- Checkpoint dir naming: epoch_X_step_Y instead of epoch_X_iter_Y
- Metric log entries use step and consumed_samples fields
- Backward compat removed (old iteration checkpoints unsupported)

tests/conftest.py

@@ -75,7 +75,7 @@ class MultiTurnDataset(Dataset):
 
 
 class EarlyStoppingDataset(Dataset):
-    """Dataset that triggers early stopping after a specified number of iterations."""
+    """Dataset that triggers early stopping after consuming a specified number of samples."""
 
     def __init__(self, length=10, stop_after=5):
         self.length = length