Custom ops with a single argument (#214)

* Add the CustomOp1 trait.

* Add an example of custom op.

* Polish the custom op example.

* Add some backward pass test for custom ops.

candle-core/tests/custom_op_tests.rs

@@ -0,0 +1,157 @@
+use candle::backend::BackendStorage;
+use candle::cpu_backend;
+use candle::{CpuStorage, CustomOp1, DType, Device, Error, Layout, Result, Shape, Tensor};
+use half::{bf16, f16};
+
+mod test_utils;
+use test_utils::to_vec1_round;
+
+fn fwd<T: num_traits::Float>(v: T, alpha: T) -> T {
+    if v.is_sign_positive() {
+        v
+    } else {
+        (v.exp() - T::one()) * alpha
+    }
+}
+
+struct Elu {
+    alpha: f64,
+}
+
+impl CustomOp1 for Elu {
+    fn name(&self) -> &'static str {
+        "elu"
+    }
+
+    fn cpu_fwd(&self, s: &CpuStorage, l: &Layout) -> Result<(CpuStorage, Shape)> {
+        use CpuStorage::*;
+
+        // In this example, we pattern match over the different dtypes. Some helper functions and
+        // traits from the `cpu_backend` module can be used to avoid this in some common cases, see
+        // e.g. `Map1`.
+        let storage = match s {
+            BF16(s) => {
+                let alpha = bf16::from_f64(self.alpha);
+                let data = cpu_backend::unary_map(s, l, |v| fwd(v, alpha));
+                BF16(data)
+            }
+            F16(s) => {
+                let alpha = f16::from_f64(self.alpha);
+                let data = cpu_backend::unary_map(s, l, |v| fwd(v, alpha));
+                F16(data)
+            }
+            F32(s) => {
+                let alpha = self.alpha as f32;
+                let data = cpu_backend::unary_map(s, l, |v| fwd(v, alpha));
+                F32(data)
+            }
+            F64(s) => {
+                let data = cpu_backend::unary_map(s, l, |v| fwd(v, self.alpha));
+                F64(data)
+            }
+            _ => Err(Error::UnsupportedDTypeForOp(s.dtype(), "elu").bt())?,
+        };
+        Ok((storage, l.shape().clone()))
+    }
+}
+
+#[test]
+fn custom_op1_no_backward() -> Result<()> {
+    let cpu = &Device::Cpu;
+    let t = Tensor::arange(0u32, 12u32, cpu)?.to_dtype(DType::F32)?;
+    let t = (t - 5.)?;
+    let elu_t = t.custom_op1(Elu { alpha: 1. })?;
+    assert_eq!(
+        to_vec1_round(&elu_t, 4)?,
+        &[-0.9933, -0.9817, -0.9502, -0.8647, -0.6321, 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0]
+    );
+    Ok(())
+}
+
+// Define a similar struct as Elu but with backward support.
+fn bwd<T: num_traits::Float>(v: T, alpha: T) -> T {
+    if v.is_sign_positive() {
+        T::one()
+    } else {
+        v.exp() * alpha
+    }
+}
+
+struct EluBackward {
+    alpha: f64,
+}
+
+impl CustomOp1 for EluBackward {
+    fn name(&self) -> &'static str {
+        "elu-bwd"
+    }
+
+    fn cpu_fwd(&self, s: &CpuStorage, l: &Layout) -> Result<(CpuStorage, Shape)> {
+        use CpuStorage::*;
+
+        // In this example, we pattern match over the different dtypes. Some helper functions and
+        // traits from the `cpu_backend` module can be used to avoid this in some common cases, see
+        // e.g. `Map1`.
+        let storage = match s {
+            BF16(s) => {
+                let alpha = bf16::from_f64(self.alpha);
+                let data = cpu_backend::unary_map(s, l, |v| bwd(v, alpha));
+                BF16(data)
+            }
+            F16(s) => {
+                let alpha = f16::from_f64(self.alpha);
+                let data = cpu_backend::unary_map(s, l, |v| bwd(v, alpha));
+                F16(data)
+            }
+            F32(s) => {
+                let alpha = self.alpha as f32;
+                let data = cpu_backend::unary_map(s, l, |v| bwd(v, alpha));
+                F32(data)
+            }
+            F64(s) => {
+                let data = cpu_backend::unary_map(s, l, |v| bwd(v, self.alpha));
+                F64(data)
+            }
+            _ => Err(Error::UnsupportedDTypeForOp(s.dtype(), "elu").bt())?,
+        };
+        Ok((storage, l.shape().clone()))
+    }
+}
+
+struct EluWithBackward(Elu);
+
+impl EluWithBackward {
+    fn new(alpha: f64) -> Self {
+        Self(Elu { alpha })
+    }
+}
+
+impl CustomOp1 for EluWithBackward {
+    fn name(&self) -> &'static str {
+        "elu"
+    }
+
+    fn cpu_fwd(&self, s: &CpuStorage, l: &Layout) -> Result<(CpuStorage, Shape)> {
+        self.0.cpu_fwd(s, l)
+    }
+
+    fn bwd(&self, arg: &Tensor, _res: &Tensor, grad_res: &Tensor) -> Result<Tensor> {
+        let alpha = self.0.alpha;
+        let bwd = arg.custom_op1(EluBackward { alpha })?;
+        grad_res.mul(&bwd)
+    }
+}
+
+#[test]
+fn custom_op1_with_backward() -> Result<()> {
+    let cpu = &Device::Cpu;
+    let t = candle::Var::new(&[-2f32, 0f32, 2f32], cpu)?;
+    let elu_t = t.custom_op1(EluWithBackward::new(2.))?;
+    assert_eq!(to_vec1_round(&elu_t, 4)?, &[-1.7293, 0.0, 2.0]);
+
+    let grads = elu_t.backward()?;
+    let grad_x = grads.get(&t).unwrap();
+    assert_eq!(to_vec1_round(grad_x, 4)?, [0.2707, 1.0, 1.0]);
+
+    Ok(())
+}