Add the SGD optimizer (#160)

* Add the nn::optim and some conversion traits.

* Add the backward_step function for SGD.

* Get the SGD optimizer to work and add a test.

* Make the test slighly simpler.

candle-nn/tests/optim.rs

@@ -0,0 +1,19 @@
+#[cfg(feature = "mkl")]
+extern crate intel_mkl_src;
+
+use anyhow::Result;
+use candle::{Device, Var};
+use candle_nn::SGD;
+
+#[test]
+fn sgd_optim() -> Result<()> {
+    let x = Var::new(0f32, &Device::Cpu)?;
+    let sgd = SGD::new(&[&x], 0.1);
+    let xt = x.as_tensor();
+    for _step in 0..100 {
+        let loss = ((xt - 4.2)? * (xt - 4.2)?)?;
+        sgd.backward_step(&loss)?
+    }
+    assert_eq!(x.to_scalar::<f32>()?, 4.199999);
+    Ok(())
+}