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-wasm-examples/llama2-c/src/model.rs

@@ -1,5 +1,5 @@
 use candle::{DType, Device, IndexOp, Result, Tensor, D};
-use candle_nn::{rms_norm, Embedding, Linear, RmsNorm, VarBuilder};
+use candle_nn::{rms_norm, Embedding, Linear, Module, RmsNorm, VarBuilder};
 use std::collections::HashMap;
 use std::sync::{Arc, Mutex};