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-core/src/lib.rs

@@ -36,6 +36,7 @@
 mod backend;
 mod backprop;
 mod conv;
+mod convert;
 mod cpu_backend;
 #[cfg(feature = "cuda")]
 mod cuda_backend;