Add Binary Cross Entropy With Logit Loss to nn crate (#1157)

* add bce with logit loss

* add bce with logit loss

* remove imports

* fix tiny bug

* add test documentation and refactor function

* fix test cases and formatting

candle-nn/src/loss.rs

@@ -48,3 +48,25 @@ pub fn cross_entropy(inp: &Tensor, target: &Tensor) -> Result<Tensor> {
 pub fn mse(inp: &Tensor, target: &Tensor) -> Result<Tensor> {
     (inp - target)?.sqr()?.mean_all()
 }
+
+/// The binary cross-entropy with logit loss.
+///
+/// Arguments
+///
+/// * [inp]: The input tensor of dimensions `N, C` where `N` is the batch size and `C` the number
+///          of categories. This is expected to raw logits.
+/// * [target]: The ground truth labels as a tensor of u32 of dimension `N, C` where `N` is the batch size and `C` the number
+///          of categories.
+///
+/// The resulting tensor is a scalar containing the average value over the batch.
+pub fn binary_cross_entropy_with_logit(inp: &Tensor, target: &Tensor) -> Result<Tensor> {
+    let inp = crate::ops::sigmoid(inp)?;
+
+    let left_side = target * inp.log()?;
+    let right_side = (target.affine(-1., 1.))? * inp.affine(-1., 1.)?.log()?;
+
+    let loss = left_side? + right_side?;
+    let loss = loss?.neg()?.mean_all()?;
+
+    Ok(loss)
+}

candle-nn/tests/loss.rs

@@ -39,3 +39,50 @@ fn nll_and_cross_entropy() -> Result<()> {
     assert_eq!(to_vec0_round(&loss, 4)?, 1.1312);
     Ok(())
 }
+
+/* Equivalent python code:
+import torch
+import torch.nn.functional as F
+
+inp = torch.Tensor([[ 2.3611, -0.8813, -0.5006, -0.2178],
+        [ 0.0419,  0.0763, -1.0457, -1.6692],
+        [-1.0494,  0.8111,  1.5723,  1.2315],
+        [ 1.3081,  0.6641,  1.1802, -0.2547],
+        [ 0.5292,  0.7636,  0.3692, -0.8318]])
+
+target = torch.Tensor([[0., 1., 0., 0.],
+        [0., 1., 0., 0.],
+        [0., 0., 0., 1.],
+        [1., 0., 0., 0.],
+        [0., 0., 1., 0.]])
+
+print(F.binary_cross_entropy_with_logits(inp, target))
+*/
+#[test]
+fn binary_cross_entropy_with_logit() -> Result<()> {
+    let cpu = Device::Cpu;
+
+    let inp = [
+        [2.3611f32, -0.8813, -0.5006, -0.2178],
+        [0.0419, 0.0763, -1.0457, -1.6692],
+        [-1.0494, 0.8111, 1.5723, 1.2315],
+        [1.3081, 0.6641, 1.1802, -0.2547],
+        [0.5292, 0.7636, 0.3692, -0.8318],
+    ];
+
+    let target = [
+        [0.0f32, 1., 0., 0.],
+        [0., 1., 0., 0.],
+        [0., 0., 0., 1.],
+        [1., 0., 0., 0.],
+        [0., 0., 1., 0.],
+    ];
+
+    let inp = Tensor::new(&inp, &cpu)?;
+    let target = Tensor::new(&target, &cpu)?;
+
+    let loss = candle_nn::loss::binary_cross_entropy_with_logit(&inp, &target)?;
+
+    assert_eq!(to_vec0_round(&loss, 4)?, 0.8224);
+    Ok(())
+}