refactor : metric 日志改为以 optimizer step 为单位，默认每步记录

- log_interval 默认 100 -> 1，语义从 batch iteration 改为 optimizer step
- step 指标从 on_batch_end 移到 on_optimizer_step，不受梯度累积影响
- JSONL 条目新增 step 字段，保留 iter
- flush 落盘仍在 on_batch_end

astrai/config/train_config.py

@@ -68,8 +68,8 @@ class TrainConfig(BaseConfig):
         default="./checkpoint/logs", metadata={"help": "Directory for metric logs."}
     )
     log_interval: int = field(
-        default=100,
-        metadata={"help": "Number of batch iterations between metric logs."},
+        default=1,
+        metadata={"help": "Number of optimizer steps between metric logs."},
     )
     metrics: List[str] = field(
         default_factory=lambda: ["loss", "lr", "grad_norm"],