Add tanh. (#675)

* Add tanh.

* Use tanh in the lstm block.

* Add a test for tanh forward and backward passes.

candle-core/tests/grad_tests.rs

@@ -183,6 +183,15 @@ fn unary_grad(device: &Device) -> Result<()> {
         test_utils::to_vec1_round(grad_x, 2)?,
         [12.99, 2.5, 20.0, 0.15]
     );
+
+    let y = x.tanh()?;
+    let grads = y.backward()?;
+    let grad_x = grads.get(&x).context("no grad for x")?;
+    assert_eq!(test_utils::to_vec1_round(&y, 2)?, [1.0, 0.76, 1.0, 0.15]);
+    assert_eq!(
+        test_utils::to_vec1_round(grad_x, 2)?,
+        [0.01, 0.42, 0.0, 0.98],
+    );
     Ok(())
 }