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/quantized/main.rs

@@ -7,7 +7,7 @@ use tokenizers::Tokenizer;
 use candle::quantized::ggml_file::Content;
 use candle::quantized::QTensor;
 use candle::{DType, Device, IndexOp, Result, Tensor, D};
-use candle_nn::Embedding;
+use candle_nn::{Embedding, Module};
 use candle_transformers::generation::LogitsProcessor;
 
 const MAX_SEQ_LEN: usize = 4096;