refactor: merge validation into MetricCallback, simplify progress bar to optimizer steps

- Remove separate ValidationCallback, merge into MetricCallback
- Progress bar now tracks optimizer steps instead of micro-steps
- Remove unused log_interval config field and CLI flag
- Fix validation all_reduce: use SUM(loss, count) instead of AVG
- Simplify metric logging: always log every optimizer step
- Add grad_norm display to progress bar

astrai/config/train_config.py

@@ -71,10 +71,6 @@ class TrainConfig(BaseConfig):
     log_dir: str = field(
         default="./checkpoint/logs", metadata={"help": "Directory for metric logs."}
     )
-    log_interval: int = field(
-        default=1,
-        metadata={"help": "Number of optimizer steps between metric logs."},
-    )
     metrics: List[str] = field(
         default_factory=lambda: ["loss", "lr", "grad_norm"],
         metadata={"help": "Metrics to record during training."},