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

scripts/tools/train.py

@@ -159,12 +159,6 @@ def parse_args() -> argparse.Namespace:
         default="checkpoint/logs",
         help="Directory for metric logs.",
     )
-    parser.add_argument(
-        "--log_interval",
-        type=int,
-        default=1,
-        help="Number of optimizer steps between metric logs.",
-    )
     parser.add_argument(
         "--grpo_sync_interval",
         type=int,
@@ -329,7 +323,6 @@ def train(
     val_step: int,
     metrics: list[str],
     log_dir: str,
-    log_interval: int,
     dpo_beta: float,
     grpo_clip_eps: float,
     grpo_kl_coef: float,
@@ -465,7 +458,6 @@ def train(
         val_step=val_step,
         metrics=metrics,
         log_dir=log_dir,
-        log_interval=log_interval,
         gradient_checkpointing_modules=grad_ckpt_modules,
         executor_kwargs=executor_kwargs,
         extra_kwargs=strategy_kwargs,