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

* Share the Embedding and Activation parts.

* Tweak some activations.

candle-nn/src/embedding.rs

@@ -0,0 +1,29 @@
+use candle::{Result, Tensor};
+
+#[derive(Debug)]
+pub struct Embedding {
+    embeddings: Tensor,
+    hidden_size: usize,
+}
+
+impl Embedding {
+    pub fn new(embeddings: Tensor, hidden_size: usize) -> Self {
+        Self {
+            embeddings,
+            hidden_size,
+        }
+    }
+
+    pub fn embeddings(&self) -> &Tensor {
+        &self.embeddings
+    }
+
+    pub fn forward(&self, indexes: &Tensor) -> Result<Tensor> {
+        let mut final_dims = indexes.dims().to_vec();
+        final_dims.push(self.hidden_size);
+        let indexes = indexes.flatten_all()?;
+        let values = Tensor::embedding(&indexes, &self.embeddings)?;
+        let values = values.reshape(final_dims)?;
+        Ok(values)
+    }
+}