Feat: Add weights argument in BCE Loss and tests (#620)

python/mlx/nn/losses.py

@@ -117,6 +117,7 @@ def cross_entropy(
 def binary_cross_entropy(
     inputs: mx.array,
     targets: mx.array,
+    weights: mx.array = None,
     with_logits: bool = True,
     reduction: Reduction = "mean",
 ) -> mx.array:
@@ -128,6 +129,7 @@ def binary_cross_entropy(
             ``inputs`` are unnormalized logits. Otherwise, ``inputs`` are probabilities.
         targets (array): The binary target values in {0, 1}.
         with_logits (bool, optional): Whether ``inputs`` are logits. Default: ``True``.
+        weights (array, optional): Optional weights for each target. Default: ``None``.
         reduction (str, optional): Specifies the reduction to apply to the output:
           ``'none'`` | ``'mean'`` | ``'sum'``. Default: ``'mean'``.
 
@@ -159,6 +161,15 @@ def binary_cross_entropy(
     else:
         loss = -(targets * mx.log(inputs) + (1 - targets) * mx.log(1 - inputs))
 
+    # Apply weights if provided
+    if weights is not None:
+        if weights.shape != loss.shape:
+            raise ValueError(
+                f"Weights with shape {weights.shape} is not the same as "
+                f"output loss with shape {loss.shape}."
+            )
+        loss *= weights
+
     return _reduce(loss, reduction)
 
 

python/tests/test_losses.py

@@ -92,6 +92,14 @@ class TestLosses(mlx_tests.MLXTestCase):
             expected_sum = mx.sum(expected_none)
             self.assertEqual(losses_sum, expected_sum)
 
+            # With weights, no label smoothing
+            weights = mx.array([1.0, 2.0, 1.0, 2.0])
+            expected = mx.array([0.747215, 1.62186, 0.262365, 0.672944])
+            loss = nn.losses.binary_cross_entropy(
+                logits, targets, weights=weights, reduction="none"
+            )
+            self.assertTrue(mx.allclose(loss, expected))
+
         def _test_probs_as_inputs():
             probs = mx.array([0.5, 0.6, 0.7, 0.8])
             targets = mx.array([0, 0, 1, 1])