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-nn/tests/optim.rs

@@ -6,7 +6,7 @@ use test_utils::{to_vec0_round, to_vec2_round};
 
 use anyhow::Result;
 use candle::{Device, Tensor, Var};
-use candle_nn::{AdamW, Linear, ParamsAdamW, SGD};
+use candle_nn::{AdamW, Linear, Module, ParamsAdamW, SGD};
 
 #[test]
 fn sgd_optim() -> Result<()> {