fix: 修复训练循环 step/backward 顺序，重构为三重循环嵌套

- 训练循环改用 itertools.batched 实现 epoch→step→batch 三重嵌套
- on_step_begin 包裹 batch 循环，on_step_end 后接 optimizer.step/scheduler.step
- 修复首次 iteration=0 时 optimizer.step() 在 backward 之前触发的 bug
- GradientClippingCallback 改为 on_step_end（梯度已累积，step 前裁剪）
- SchedulerCallback 移除，schduler.step 由 trainer 在 optimizer.step 后直接调用
- metric_util 提取 _grad_stat 公共 helper，if param.grad: 修正为 is not None

astrai/trainer/metric_util.py

@@ -1,75 +1,42 @@
-from typing import Dict
+from typing import Any, Callable, Dict
 
+import torch
 import torch.nn as nn
 
 
-def grad_norm(model: nn.Module, norm_type: int = 2) -> Dict[str, float]:
-    """Compute gradient norm for each parameter in the model."""
-    norms = {}
+def _grad_stat(
+    model: nn.Module, fn: Callable[[torch.Tensor], Any], default: Any
+) -> dict:
+    results = {}
     for name, param in model.named_parameters():
-        norms[name] = 0.0
-        if param.grad:
-            norm = param.grad.data.norm(norm_type).item()
-            norms[name] = norm
-    return norms
+        results[name] = default
+        if param.grad is not None:
+            results[name] = fn(param.grad.data)
+    return results
+
+
+def grad_norm(model: nn.Module, norm_type: int = 2) -> Dict[str, float]:
+    return _grad_stat(model, lambda g: g.norm(norm_type).item(), 0.0)
 
 
 def grad_std(model: nn.Module) -> Dict[str, float]:
-    """Compute standard deviation of gradients for each parameter."""
-    stds = {}
-    for name, param in model.named_parameters():
-        stds[name] = 0.0
-        if param.grad:
-            std = param.grad.data.std().item()
-            stds[name] = std
-    return stds
+    return _grad_stat(model, lambda g: g.std().item(), 0.0)
 
 
 def grad_max(model: nn.Module) -> Dict[str, float]:
-    """Find the maximum absolute gradient value for each parameter."""
-    max_vals = {}
-    for name, param in model.named_parameters():
-        max_vals[name] = -float("inf")
-        if param.grad:
-            max_val = param.grad.data.max().item()
-            max_vals[name] = max_val
-
-    return max_vals
+    return _grad_stat(model, lambda g: g.max().item(), -float("inf"))
 
 
 def grad_min(model: nn.Module) -> Dict[str, float]:
-    """Find the minimum absolute gradient value for each parameter."""
-    min_vals = {}
-    for name, param in model.named_parameters():
-        min_vals[name] = float("inf")
-        if param.grad:
-            min_val = param.grad.data.min().item()
-            min_vals[name] = min_val
-
-    return min_vals
+    return _grad_stat(model, lambda g: g.min().item(), float("inf"))
 
 
 def grad_mean(model: nn.Module) -> Dict[str, float]:
-    """Compute mean of gradients for each parameter."""
-    means = {}
-    for name, param in model.named_parameters():
-        means[name] = 0.0
-        if param.grad:
-            mean = param.grad.data.mean().item()
-            means[name] = mean
-
-    return means
+    return _grad_stat(model, lambda g: g.mean().item(), 0.0)
 
 
 def grad_nan_num(model: nn.Module) -> Dict[str, int]:
-    """Count the number of NaNs in gradients for each parameter."""
-    nan_nums = {}
-    for name, param in model.named_parameters():
-        nan_nums[name] = 0
-        if param.grad:
-            nan_num = param.grad.isnan().sum().item()
-            nan_nums[name] = nan_num
-    return nan_nums
+    return _grad_stat(model, lambda g: g.isnan().sum().item(), 0)
 
 
 def ctx_get_loss(ctx):