Add a simple Module trait and implement it for the various nn layers (#500)

* Start adding the module trait.

* Use the module trait.

* Implement module for qmatmul.

candle-examples/examples/stable-diffusion/utils.rs

@@ -1,4 +1,5 @@
 use candle::{Device, Result, Tensor};
+use candle_nn::Module;
 
 pub fn linspace(start: f64, stop: f64, steps: usize) -> Result<Tensor> {
     if steps < 1 {