added mse_loss, nll_loss and kl_div_loss (#98)

* added mse_loss, nll_loss and kl_div_loss

* fixed axis not defined error in nll_loss

* fixed axis not defined in kl_div_loss

* added tests for mse, nll and kl_div

* modified docstrings and added reduce helper func

* updated docstring in kl_div_loss and moved helper func

* added new kl divergence implementation

* added reduction to test

* updated docstring of kl_div_loss with correct spelling

* added losses to nn.rst in docs

python/mlx/nn/losses.py

@@ -2,10 +2,9 @@
 
 import mlx.core as mx
 
-
 def cross_entropy(
     logits: mx.array, targets: mx.array, axis: int = -1, reduction: str = "none"
-):
+) -> mx.array:
     """
     Computes the cross entropy loss between logits and targets.
 
@@ -22,6 +21,84 @@ def cross_entropy(
     score = mx.take_along_axis(logits, targets[..., None], axis).squeeze(-1)
     loss = mx.logsumexp(logits, axis=axis) - score
 
+    return _reduce(loss, reduction)
+
+
+def l1_loss(predictions: mx.array, targets: mx.array, reduction: str = "none") -> mx.array:
+    """
+    Computes the L1 loss between predictions and targets.
+
+    Args:
+        predictions (mx.array): The predicted values.
+        targets (mx.array): The target values.
+        reduction (str, optional): Specifies the reduction to apply to the output:
+          ``'none'`` | ``'mean'`` | ``'sum'``. Default: ``'none'``.
+
+    Returns:
+        mx.array: The computed L1 loss.
+    """
+    loss = mx.mean(mx.abs(predictions - targets))
+    
+    return _reduce(loss, reduction)
+
+
+def mse_loss(predictions: mx.array, targets: mx.array, axis: int = -1, reduction: str = "none") -> mx.array:
+    """
+    Computes the mean squared error loss between predictions and targets.
+
+    Args:
+        predictions (mx.array): The predicted values.
+        targets (mx.array): The target values.
+        axis (int, optional): The axis over which to compute softmax. Default: ``-1``.
+        reduction (str, optional): Specifies the reduction to apply to the output:
+          ``'none'`` | ``'mean'`` | ``'sum'``. Default: ``'none'``.
+
+    Returns:
+        mx.array: The computed mean squared error loss.
+    """
+    loss = mx.mean(mx.square(predictions - targets), axis)
+    
+    return _reduce(loss, reduction)
+
+
+def nll_loss(logits: mx.array, targets: mx.array, axis: int = -1, reduction: str = "none") -> mx.array:
+    """
+    Computes the negative log likelihood loss between logits and targets.
+
+    Args:
+        logits (mx.array): The predicted logits.
+        targets (mx.array): The target values.
+        axis (int, optional): The axis over which to compute softmax. Default: ``-1``.
+        reduction (str, optional): Specifies the reduction to apply to the output:
+          ``'none'`` | ``'mean'`` | ``'sum'``. Default: ``'none'``.
+
+    Returns:
+        mx.array: The computed NLL loss.
+    """
+    loss = -mx.take_along_axis(logits, targets[..., None], axis).squeeze(-1)
+
+    return _reduce(loss, reduction)
+
+
+def kl_div_loss(logits: mx.array, targets: mx.array, axis: int = -1, reduction: str = "none") -> mx.array:
+    """
+    Computes the Kullback-Leibler divergence loss between logits and targets.
+
+    Args:
+        logits (mx.array): Logits for the distribution p.
+        targets (mx.array): Log probabilities for the distribution q.
+        axis (int, optional): The axis over which to compute softmax. Default: ``-1``.
+        reduction (str, optional): Specifies the reduction to apply to the output:
+          ``'none'`` | ``'mean'`` | ``'sum'``. Default: ``'none'``.
+
+    Returns:
+        mx.array: The computed Kullback-Leibler divergence loss.
+    """
+    loss = mx.sum(mx.exp(targets) * (targets - logits), axis)
+
+    return _reduce(loss, reduction)
+
+def _reduce(loss: mx.array, reduction: str = 'none'):
     if reduction == "mean":
         return mx.mean(loss)
     elif reduction == "sum":
@@ -30,17 +107,3 @@ def cross_entropy(
         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))