[nn] Move the Embedding and Activation parts. (#116)

* Share the Embedding and Activation parts.

* Tweak some activations.

candle-nn/src/activation.rs

@@ -0,0 +1,18 @@
+use candle::Tensor;
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum Activation {
+    Gelu,
+    Relu,
+    Elu(f64),
+}
+
+impl Activation {
+    pub fn forward(&self, xs: &Tensor) -> candle::Result<Tensor> {
+        match self {
+            Self::Gelu => xs.gelu(),
+            Self::Relu => xs.relu(),
+            &Self::Elu(alpha) => xs.elu(alpha),
+        }
+    }
+}