feat: 训练中新增验证循环

- TrainConfig 添加 val_dataset/val_step 字段
- TrainContext 添加 val_dataloader/val_loss 字段
- 新增 ValidationCallback 按 step 触发验证 + 训练结束时验证
- ProgressBar/MetricLogger 支持 val_loss 展示与记录

astrai/trainer/train_context.py

@@ -26,6 +26,8 @@ class TrainContext:
     epoch: int = field(default=0)
     iteration: int = field(default=0)
     loss: float = field(default=0.0)
+    val_dataloader: DataLoader = field(default=None)
+    val_loss: float = field(default=0.0)
 
     world_size: int = field(default=1)
     rank: int = field(default=0)
@@ -88,6 +90,23 @@ class TrainContextBuilder:
             prefetch_factor=cfg.prefetch_factor,
         )
 
+        if cfg.val_dataset is not None:
+            val_sampler = ResumableDistributedSampler(
+                data_source=cfg.val_dataset,
+                start_epoch=0,
+                start_iter=0,
+                seed=cfg.random_seed,
+                shuffle=False,
+            )
+            context.val_dataloader = DataLoader(
+                cfg.val_dataset,
+                batch_size=cfg.batch_per_device,
+                sampler=val_sampler,
+                num_workers=cfg.num_workers,
+                pin_memory=cfg.pin_memory,
+                prefetch_factor=cfg.prefetch_factor,
+            )
+
         context.strategy = StrategyFactory.create(
             model=context.model,
             train_type=self.config.strategy,