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-core/src/quantized/mod.rs

@@ -184,6 +184,7 @@ impl QTensor {
     }
 }
 
+#[derive(Debug)]
 pub struct QMatMul(QTensor);
 
 impl QMatMul {