MLE and L1 loss functions (#88)

* MLE and L1 loss functions

* logsoftmax change and tests

* subtract max logit for numerical stability

* l1 name change

* cross entropy reduction + unit tests

* docstrings

* l1 test name change

* old loss impl + default none

python/mlx/nn/losses.py

@@ -2,7 +2,45 @@
 
 import mlx.core as mx
 
+def cross_entropy(logits: mx.array, targets: mx.array, axis: int = -1, reduction: str = 'none'):
+    """
+    Computes the cross entropy loss between logits and targets.
 
-def cross_entropy(logits: mx.array, targets: mx.array, axis: int = -1):
+    Args:
+        logits (mx.array): The predicted logits.
+        targets (mx.array): The target values.
+        axis (int, optional): The axis over which to compute softmax. Defaults to -1.
+        reduction (str, optional): Specifies the reduction to apply to the output: 'none' | 'mean' | 'sum'. 
+                                    'none': no reduction will be applied. 
+                                    'mean': the sum of the output will be divided by the number of elements in the output.
+                                    'sum': the output will be summed. 
+                                    Defaults to 'none'.
+
+    Returns:
+        mx.array: The computed cross entropy loss.
+    """
     score = mx.take_along_axis(logits, targets[..., None], axis).squeeze(-1)
-    return mx.logsumexp(logits, axis=axis) - score
+    loss = mx.logsumexp(logits, axis=axis) - score
+
+    if reduction == 'mean':
+        return mx.mean(loss)
+    elif reduction == 'sum':
+        return mx.sum(loss)
+    elif reduction == 'none':
+        return loss
+    else:
+        raise ValueError("Invalid reduction. Must be 'none', 'mean', or 'sum'.")
+
+def l1_loss(predictions: mx.array, targets: mx.array):
+    """
+    Computes the L1 loss between predictions and targets.
+
+    Args:
+        predictions (mx.array): The predicted values.
+        targets (mx.array): The target values.
+
+    Returns:
+        mx.array: The computed L1 loss.
+    """
+    return mx.mean(mx.abs(predictions - targets))
+

python/tests/test_nn.py

@@ -10,7 +10,6 @@ import mlx_tests
 import numpy as np
 from mlx.utils import tree_flatten, tree_map, tree_unflatten
 
-
 class TestNN(mlx_tests.MLXTestCase):
     def test_linear(self):
         inputs = mx.zeros((10, 4))
@@ -21,8 +20,27 @@ class TestNN(mlx_tests.MLXTestCase):
     def test_cross_entropy(self):
         logits = mx.array([[0.0, -float("inf")], [-float("inf"), 0.0]])
         targets = mx.array([0, 1])
-        losses = nn.losses.cross_entropy(logits, targets)
+
-        self.assertTrue(mx.array_equal(losses, mx.zeros((2,))))
+        # Test with reduction 'none'
+        losses_none = nn.losses.cross_entropy(logits, targets, reduction='none')
+        expected_none = mx.array([0.0, 0.0])
+        self.assertTrue(mx.array_equal(losses_none, expected_none))
+
+        # Test with reduction 'mean'
+        losses_mean = nn.losses.cross_entropy(logits, targets, reduction='mean')
+        expected_mean = mx.mean(expected_none)
+        self.assertEqual(losses_mean, expected_mean)
+
+        # Test with reduction 'sum'
+        losses_sum = nn.losses.cross_entropy(logits, targets, reduction='sum')
+        expected_sum = mx.sum(expected_none)
+        self.assertEqual(losses_sum, expected_sum)
+
+    def test_l1_loss(self):
+        predictions = mx.array([0.5, 0.2, 0.9, 0.0])
+        targets = mx.array([0.5, 0.2, 0.9, 0.0])
+        losses = nn.losses.l1_loss(predictions, targets)
+        self.assertEqual(losses, 0.0)
 
     def test_gelu(self):
         inputs = [1.15286231, -0.81037411, 0.35816911, 0.77484438, 0.66276414]