Add tanh. (#675)

* Add tanh.

* Use tanh in the lstm block.

* Add a test for tanh forward and backward passes.

candle-core/src/tensor.rs

@@ -460,6 +460,7 @@ impl Tensor {
     unary_op!(log, Log);
     unary_op!(sin, Sin);
     unary_op!(cos, Cos);
+    unary_op!(tanh, Tanh);
     unary_op!(abs, Abs);
     unary_op!(sqr, Sqr);
     unary_op!(sqrt, Sqrt);