!30628 Add CPU support for BCEWithLogitsLoss

Merge pull request !30628 from 吕昱峰（Nate.River）/bceloss

mindspore/python/mindspore/nn/loss/loss.py

@@ -1251,7 +1251,7 @@ class BCEWithLogitsLoss(LossBase):
         ValueError: If `reduction` is not one of 'none', 'mean', 'sum'.
 
     Supported Platforms:
-        ``Ascend``  ``GPU``
+        ``Ascend``  ``GPU``  ``CPU``
 
     Examples:
         >>> logits = Tensor(np.array([[-0.8, 1.2, 0.7], [-0.1, -0.4, 0.7]]).astype(np.float32))
@@ -1265,6 +1265,7 @@ class BCEWithLogitsLoss(LossBase):
     def __init__(self, reduction='mean', weight=None, pos_weight=None):
         """Initialize BCEWithLogitsLoss."""
         super(BCEWithLogitsLoss, self).__init__()
+        self.reduction = reduction
         self.bce_with_logits_loss = P.BCEWithLogitsLoss(reduction=reduction)
         if isinstance(weight, Parameter):
             raise TypeError(f"For '{self.cls_name}', the 'weight' can not be a Parameter.")
@@ -1273,10 +1274,42 @@ class BCEWithLogitsLoss(LossBase):
         self.weight = weight
         self.pos_weight = pos_weight
         self.ones = P.OnesLike()
+        self.is_cpu = context.get_context("device_target") == "CPU"
+
+    def _construct_cpu(self, logits, labels):
+        """Use native implementation for CPU."""
+        max_val = F.maximum(-logits, 0)
+
+        if self.pos_weight is not None:
+            log_weight = ((self.pos_weight - 1) * labels) + 1
+            loss = (1 - labels) * logits
+            loss_1 = F.log(F.exp(F.neg_tensor(max_val)) + F.exp(F.neg_tensor(logits) - max_val)) + max_val
+            loss += log_weight * loss_1
+        else:
+            loss = (1 - labels) * logits
+            loss += max_val
+            loss += F.log(F.exp(F.neg_tensor(max_val)) + F.exp(F.neg_tensor(logits) - max_val))
+
+        if self.weight is not None:
+            output = loss * self.weight
+        else:
+            output = loss
+
+        if self.reduction == "mean":
+            return F.reduce_mean(output)
+        if self.reduction == "sum":
+            return F.reduce_sum(output)
+        return output
 
     def construct(self, logits, labels):
         _check_is_tensor('logits', logits, self.cls_name)
         _check_is_tensor('labels', labels, self.cls_name)
+        if self.is_cpu:
+            return self._construct_cpu(logits, labels)
+        return self._construct_gpu_ascend(logits, labels)
+
+    def _construct_gpu_ascend(self, logits, labels):
+        """Use P.BCEWithLogitsLoss for Ascend and GPU."""
         ones_input = self.ones(logits)
         if self.weight is not None:
             weight = self.weight