Add huber loss support

Sources/TensorFlow/Loss.swift

@@ -290,3 +290,31 @@ public func sigmoidCrossEntropy<Scalar: TensorFlowFloatingPoint>(
     let negAbsLogits = max(logits, -logits) // Custom `abs` to compute gradients at `0`.
     return reduction(maxLogitsWithZero - logits * labels + log1p(exp(-negAbsLogits)))
 }
+
+/// Returns the Huber loss between predictions and expectations.
+///
+/// For each value `x` in the difference `expected - predicted`, the loss is:
+/// - `0.5 * x^2` if `abs(x) <= δ`.
+/// - `0.5 * δ^2 + δ * (|x| - δ)` otherwise.
+///
+/// - Source: [Uncyclopedia article](https://en.wikipedia.org/wiki/Huber_loss).
+///
+/// - Parameters:
+///   - predicted: Predicted outputs from a neural network.
+///   - expected: Expected values, i.e. targets, that correspond to the correct output.
+///   - delta: A floating point scalar representing the point where the Huber loss function changes
+///     from quadratic to linear.
+///   - reduction: Reduction to apply on the computed element-wise loss values.
+@differentiable(wrt: predicted)
+public func huberLoss<Scalar: TensorFlowFloatingPoint>(
+    predicted: Tensor<Scalar>,
+    expected: Tensor<Scalar>,
+    delta: Scalar,
+    reduction: @differentiable (Tensor<Scalar>) -> Tensor<Scalar> = { $0.sum() }
+) -> Tensor<Scalar> {
+    let error = expected - predicted
+    let absError = abs(error)
+    let quadratic = min(absError, delta)
+    let linear = absError - quadratic
+    return reduction((0.5 * quadratic * quadratic) + (delta * linear))
+}

Tests/TensorFlowTests/LossTests.swift

@@ -216,6 +216,33 @@ final class LossTests: XCTestCase {
             [-0.0625, -0.01490036, 0.04759964, 0.0]])
         assertEqual(computedGradient, expectedGradient, accuracy: 1e-6)
     }
+    func testHuberLoss() {
+        let predictions = Tensor<Float>([[0.9, 0.2, 0.2], [0.8, 0.4, 0.6]])
+        let labels = Tensor<Float>([[1, 0, 1], [1, 0, 0]])
+
+        do {
+          // Test adapted from:
+          // https://github.com/tensorflow/tensorflow/blob/148f07323f97ef54998f28cd95c195064ce2c426/tensorflow/python/keras/losses_test.py#L1554
+          let loss = huberLoss(predicted: predictions, expected: predictions, delta: 1)
+          assertEqual(loss, Tensor(0), accuracy: 1e-6)
+        }
+
+        do {
+          // Test adapted from:
+          // https://github.com/tensorflow/tensorflow/blob/148f07323f97ef54998f28cd95c195064ce2c426/tensorflow/python/keras/losses_test.py#L1560
+          // The expected loss was computed using Python TensorFlow 2.0.0-beta1:
+          // ```
+          // import tensorflow as tf # 2.0.0-beta1
+          // predictions = tf.constant([[0.9, 0.2, 0.2], [0.8, 0.4, 0.6]])
+          // labels = tf.constant([[1.0, 0.0, 1.0], [1.0, 0.0, 0.0]])
+          // loss = tf.losses.Huber(delta=1.0, reduction=tf.losses.Reduction.SUM)
+          // print(loss(labels, predictions))
+          // # tf.Tensor(0.62500006, shape=(), dtype=float32)
+          // ```
+          let loss = huberLoss(predicted: predictions, expected: labels, delta: Float(1))
+          assertEqual(loss, Tensor(0.62500006), accuracy: 1e-6)
+        }
+    }
 
     static var allTests = [
         ("testL1Loss", testL1Loss),
@@ -237,5 +264,6 @@ final class LossTests: XCTestCase {
          testSoftmaxCrossEntropyWithProbabilitiesGrad),
         ("testSigmoidCrossEntropyLoss", testSigmoidCrossEntropyLoss),
         ("testSigmoidCrossEntropyGradient", testSigmoidCrossEntropyGradient),
+        ("testHuberLoss", testHuberLoss)
     ]
 }