fix: focal loss

Co-authored-by: Copilot <copilot@github.com>

ginka/utils.py

@@ -71,10 +71,11 @@ def masked_focal(
     corrected = target.clone()
     corrected[~in_set] = 0
 
-    # 逆频类别权重：batch 内频率越高，权重越小
+    # 逆频类别权重：用原始 target 统计频率，避免 corrected 中人工填 0 膨胀
+    # count[0]，导致 weight[0] 趋近于 0、非专属位置损失被消除的问题
     class_weight = None
     if balance:
-        flat   = corrected.view(-1)                             # [B*S]
+        flat   = target.view(-1)                                # [B*S] 原始标签
         counts = torch.bincount(flat, minlength=C).float()     # [C]
         class_weight = flat.numel() / (counts.clamp(min=1.0) * C)
         class_weight[counts == 0] = 0.0                        # 未出现类别不参与