derivative for GELU (#1160)

* derivative for GELU

* add tests

candle-core/tests/grad_tests.rs

@@ -192,6 +192,19 @@ fn unary_grad(device: &Device) -> Result<()> {
         test_utils::to_vec1_round(grad_x, 2)?,
         [0.01, 0.42, 0.0, 0.98],
     );
+
+    // testing compared to pytorch nn.GELU(approximate = 'tanh')
+    let y = x.gelu()?;
+    let grads = y.backward()?;
+    let grad_x = grads.get(&x).context("no grad for x")?;
+    assert_eq!(
+        test_utils::to_vec1_round(&y, 4)?,
+        [2.9964, 0.8412, 3.9999, 0.0839]
+    );
+    assert_eq!(
+        test_utils::to_vec1_round(grad_x, 4)?,
+        [1.0116, 1.0830, 1.0003, 0.6188],
+    );
     Ok(())
 }